I do several things to keep the player characters interested and invested in each other.
- At the start of the game, I insist that players coordinate backgrounds (subject to my approval) such that each character know at least one, and preferably two or more, of the other characters. In general, I prefer these connections be positive; the most negative I will usually tolerate is on the order of a friendly rivalry. In general, I also prefer that if you start from any one character, you can get to any other character by following these pre-arranged links. (In graph theory language, the players are all connected, although indirect connections are fine; the alternative would be two more more sub-groups connected internally but not to each other.) And finally, I try to ensure that each connection is more than trivial, but not necessarily life-binding.
So for instance, "We met in a bar twenty years ago and never saw each other again," is trivial. "We are cousins who are best friends and we are rarely separated," is life-binding and more than I look for (although it's fine if that's what they want.) Things like the following are what I look for, and/or what I've seen in the past:
- Our characters served in the same unit years ago, and knew each other, but haven't kept in contact...
- I served as a mercenary escort once, while he was travelling with his master from here to there; along the way, this happened....
- We weathered the siege/plague/earthquake of wherever together some time back....
Now, some players are genuinely not wired that way-- if you ask them for backstory, they freeze; if you given the one, they can't connect to it. When I run into a player like that, I have to respect that, but I try very hard to get everyone to adhere to the guidelines.
That does not directly solve the problem. (It actually solves the problem of getting the characters all on the same page at the start of the game.) But it does often give me enough to work with to do the following:
- With enough insight into character backgrounds, and with overlapping backgrounds, I try to give every character an mid-term to long-term goal or plot arc, and then I try to modulate that by giving at least one other character a minor to moderate interest in how the first character's arc plays out.
It's important (to me, for the games I want to run) that these arcs not be strictly opposing: If one character has sworn blood-vengeance on an NPC, I won't give another character the goal of keeping that NPC alive. But I might give another character the goal of getting something from that NPC before his death, or getting the NPC to do something, etc.
And I also try to modulate this in another direction by giving other characters-- ideally, not the same one-- influence over the plot lines. So continuing that thought:
- Player A has sworn to kill Sir Odious, his parents' killer
- Sir Odious has information that will help Player B in her quest to do something else
- Player C knows someone who can be bribed into giving up information about where Sir Odious will be
In that way, for each of the various player sub-quests going on, at least one or two others will be involved somehow, even if only at the periphery. Ideally, Player B has some motivation for something to happen, and Player C has something he needs-- something at least moderately costly or risky. They are invested.
One thing I would not do-- at least not again-- is what you tried:
I've had players create characters (with backgrounds) completely secretly from each other with the hopes of allowing the character interaction to be heavily role-played at the table. Didn't work because of very incompatible characters.
I've never done that, specifically, but I've inadvertently done similar things and it never worked well. It seems like it should work, especially if you pattern it similar to what I've outlined above, but there's a structural weakness to it: If the players, starting out with the relative blindness of only knowing their little part of the background, they just might not see those connections you built in for them, and won't give themselves the incentive to start sharing information. And if your players were the sort that would do that naturally, you wouldn't have to go through these acrobatics in the first place.
Energy
Put focus into roleplaying, even if you're watching someone else do it. Focus yourself on what they are saying and doing, even if it's kinda boring, and project your body language and voice while you're acting in-character. Be much less high-intensity when simply describing your bonuses while rolling, or asking someone to pass the chips. This will create natural focus on the roleplaying aspect of the game.
Speak In-Character
Wherever possible, assume what others are doing is a prompt for in-character roleplaying. DM asks what you do? Turn to someone, and say 'Philius, methinks we should cross that bridge and by Everam and St George, charge those there gnolls with swords in hand. Once we have them subdued, we'll take some answers from them, so we must leave at least some alive! What say you, well-met friend?"
Even if people don't respond in-character, and instead shift it back ooc, roleplaying just happened. Keep doing it and others will soon follow.
Ham It Up
Your character is 'quiet' and 'reasonable' and NOPE. Your character is the hammiest of ham. He's a loud cliche. He is instantly identifiable as the tropes that make him up - and he defines the setting by his very presence. It's unfortunate - but humans love ham. They love it.. a lot. Be something simple and understandable and loud, and they will get with the program really fast.
You can do this by being a masterful actor and roleplayer with any character, even a non-hammy one, but it is easiest with ham, so ham I will advise. Your paladin isn't just a paladin who likes cheese and moonlit walks on the beach - he's SIR GALAHAD THE MIGHTY, SUBDUER OF THE PEASANTS, DEFENDER OF THE WOMENFOLK, AND HIS MOUSTACHE BRISTLES AT THE SLIGHTEST SUGGESTION OF DRAGONS.
'Big' traits tend to focus things on the roleplaying a lot faster. Simpler is easier for the audience to understand.
Find Allies
Find people who will respond to your dramatic offers. When you address people, address them first, so they respond in-character, and then immediately pull other people in. People ignore offers initially, but if something is already rolling, they'll get rolled in with it. Some people will instinctively resist roleplaying offers, for all kinds of reasons - learn to identify them too, and offer to them last, once the roleplaying scene has the most momentum.
Be good at plot
Being able to identify where the adventure is going will let you advance the plot during a roleplaying scene - which both speeds up the adventure and means the time spent on roleplaying won't cause a weak GM to not let you hit the end of it.
Roleplay during combat
'LOOK OUT, FARAMIR! THE GNOLL IS AT YOUR BACK!' 'Galahad charges at the gnoll attacking' moving mini 'faramir, and' rolls dice 'swings at it with his mighty sword.' By including both speech and roleplaying-description in amongst your mechanical actions, you partially negate the disconnect that happens during the mechanics-rich combat portions of sessions. Have to know what you are doing on your turn before your turn rolls around, or anti-roleplayers will complain your roleplaying is slowing things up if you are not clearly doing it faster than anyone else.
Additionally, being good at combat, and giving tactical advice in-character that leads to defeating enemies quickly, will give more time overall for non-combat-constrained roleplaying itself.
Occasionally, roleplay during others' turns - have Galahad shout an encouraging phrase at an opportune moment. This has to be rare, and well-timed, though - an advanced technique.
Be Heroic, or Dastardly
Again, ham. By being heroic, and roleplaying it hard, you make other people who are not roleplaying feel heroic. By being dastardly, and roleplaying it hard, you make other people feel heroic also who are not roleplaying. You're giving them some of your roleplaying energy in a way that feels good for them. Morally grey is, again, a tougher sell. Note this isn't 'good' or 'evil', it's more saturday morning cartoon than that. Snidely Whiplashi, or Dudley DoRight.
Incorporate the GM
Don't just roleplay at fellow players. Roleplay at NPCs. Treat them with importance, and give the GM offers to roleplay right back at you. All of this applies to the GM, too. Getting the GM on-board with roleplaying, especially if you can advance the story while doing so, will be a tremendous boon to your cause.
You're right, by the way. Premade adventures, split up groups, schedules, public venues, this stuff just kills roleplaying and really makes it quite hard - I literally could not design a better system to do so.
But even in those kind of circumstances, I have personally sparked roleplaying in some extremely tough crowds. You won't see a huge improvement - but even the tiniest bit of roleplaying can be a huge welcome to you if you're in a roleplaying drought, and if you play with regularly the same pool of people, you'll find people gravitating to you that appreciate roleplaying, perhaps even to the extent that people will fight to have you in their groups.
Overall, though, the roleplaying will be in many ways a simpler thing than the rare high level roleplaying you can get in a home group.
But it's certainly not impossible.
Just have the courage to keep trying and don't give up.
Best Answer
First of all: Don't worry. Having one player that prefers roleplaying over combat is, at least to me, pretty much the default. I think I literally have never DM'd to a table that didn't have that one Bard or Rogue that wants to sleep during combat and gets all excited when they get to a town and they can talk to people. That's normal and usually everyone can have fun together without much problem.
Comments on Session 0
So, usually, this kind of thing would be talked about in a Session 0. The "problem" with new players is that they are, well, new players. They don't know what they like or not about the system/game, because they have never played it before. It's like asking "do you like this food?" to someone who has never eaten that - or anything similar, for that matter.
And, similar to tasting food, usually tasting it only once is not enough. Even a long session like yours is still little time to get a grasp on what you actually like or dislike in the game. Maybe the player didn't like one very specific aspect of how these combats happened, but can't figure it out yet.
So... play a little more as usual and find out what the actual likes of the players.
Early levels are usually a little more boring than the rest of the game
One of the main reasons to try to keep playing is that early levels in 5e are quite boring. I am assuming you ran this session with 1st level characters, since the adventure you mention start at 1st level. The Warlock didn't get their Eldritch Invocations yet, they know 2 spells and have 1 spell slot. Their playstyle for now, as with most other classes, lacks choice. There is a reason the XP chart makes characters fly through the early levels: they are only to get a grasp of the combat for new players, and to construct your character personality.
Warlock, in particular, is a class that is very... strange. It's like a full spellcaster that has a playstyle more akin to martial classes.
So, just to reaffirm: let them find out their tastes a little longer. Ask them to try out one more "regular" session.
After that, I have seen a few possible outcomes:
D&D is combat-centric
This is both historical/cultural and designed. Monsters have a XP reward attached to them as one of the most notable numbers in their sheet, and there is almost none advice on how to reward things like talking to people and solving problems that are not encounters. The vast majority of class features are supposed to help in combat. Adventures are written in a way that most of the challenges are overcome through combat.
You can certainly play a story-driven D&D campaign, and make it combat light, but the system design makes it way harder for you to do that.
Milestones
I think this is a needed advice in case you didn't think about it yet: use Milestones to track levels, or any other alternative ways of rewarding experience. This will considerably reduce the focus of combat and tell the players they don't need to murder everything they see to get their XP share.
Finding a Middle Ground
Now, let us assume the player didn't hate combat, and the other players didn't hate roleplaying. There is a similar question here, and you can read it to get some insight. But some things you may do to find a middle ground:
The last two bullets make an important point: Combats (or, more generally, Encounters) are about a clash of motivations or goals. They don't need to start because one side wants to kill the other, and they don't need to end only when one side is dead.
Practical Example
Again going to the Bard example: consider an encounter where they finally get closer to the Tome, and there are some monsters guarding it. They don't care about killing the monsters, they care about getting the tome. Killing the monsters and then safely getting the Tome is one way to solve this, but also is becoming invisible and stealthing through the monsters and simply running away with the monsters. Maybe if they try to just walk through invisible it won't work because the monsters are very attentive and will notice the Tome missing as soon as it gets stolen and will enter a rampage mode, but if the other characters of the party distract the monsters while the bard stealthily steals the tome, and then they run away as soon as he is safe, that is as solved as killing the monsters, the Bard didn't need to enter combat, and the other players got their combats. In order to not make it trivial, the Tome is guarded by some traps that the Bard will have to overcome while the party is fighting the monsters, who are overwhelming the party. If the Bard takes too long the party will have to run or die before they manage to get the Tome and they were defeated (and, in the case of running, defeat also does not mean death: the Monsters goal is to guard the tome. Killing the adventurers is one way to do it, making them flee is another).
Although I have not run this encounter with the Bard I have been mentioning, I have done similar encounters and the party usually would have fun as a whole. By the end they would be like "Whoof you took too long I thought that Gorgon was going to kill me, I almost got petrified you know?!?!", "Excuse me I had to deactivate a fire breathing statue that burnt my beloved cloak... It was a parting gift from my master okay????", with adrenaline pumping in their blood.
Keep getting feedback
You are learning as well. It is very nice that you asked feedback from your players and that you asked advice here. Keep improving. Unless you have players on a really extreme end of the "likes" spectrum and they can't stand the way the other players have fun, you will certainly be able to find a sweet common ground where everyone has fun. Keep asking for feedback, keep finding advice on how to improve on that feedback, just... keep learning.