Maybe I'm treating the question as more specific than it needs to be, but in your example it appears to me as though player 1's agency is being denied. Twice she stated her action clearly, and yet somehow she failed to get the results of that action back from you.
You don't have to wait until all players are agreed before allowing a player to act. Now, OK, you don't want the party to split unless really necessary, but until they're actually out of earshot of each other, let each player get on with what they want to do. Running through your example hopefully will show the principles I mean:
GM: Okay, you're standing on the edge of the forest.
Player 1: Okay, I ready my bow and start forward.
GM: fine
Player 2: Hold on, I'm still talking to this other guy.
GM: Player 1, are you scouting ahead alone?
Player 1: I'll take a look, I won't go far until the others catch up.
Player 3: Did we stock up on bread? I need to put on my night vision
goggles.
GM: If the bread's not on your equipment list then you didn't stock
up [or if that's not your style of game, "yes, you stocked up on usual provisions at the supermarket yesterday"]. You now have your goggles on.
Player 4: [Crazy roleplaying thing!]
GM: [Crazy roleplaying response]
GM: Okay, are you guys going into the forest?
Player 1: I am, yeah.
GM: You start to walking into the trees...
Player 2: One sec, I need to cast detect evil.
GM: No problem, but Player 1 was already doing this while you finished with the other guy and that roleplaying thing happened.
... the undergrowth is mostly pretty thick, but you easily find what
might be an animal trail. You'll be able to travel much faster along
the trail if you want to. Player 2, if you've finished your conversation with the other guy, you cast Detect Evil. Nothing in
range. I assume you maintain it as you start into the forest?
Player 2: of course
GM: You can't see exactly where player 1 is, but she can't have got far. Okay, actions from everybody else?
Principles:
- Stay in the conversation
- Respond to actions and requests for information as quickly as possible. But in the case of Player 1 setting off ahead alone, I think it's also reasonable to give the others an opportunity to react and for Player 1 to confirm before doing something that takes a little while and might be foolish. You don't want a conversation that goes, "1: I ready my bow and start into the forest, "2: hang on a sec...", "GM: 1, you fall in a spiked pit and get ambushed by 40 goblins", "1: well, actually, I didn't mean I'd leave the party, I was about to say I'd have waited when 2 said to hang on but you interrupted", "GM: Ah. I totally didn't get that from what you said".
- If something can happen, narrate it happening.
- You don't need to design the adventure specially to avoid delays. Once the players get used to the style, they can keep the action going just by acting. Or they can stop when they really need to, they're in control.
- Don't ask the players what's happening. Ask the players what they're doing, tell them what's happening.
Now, there's a whole other scenario you have to be able to deal with:
GM: Okay, you're standing on the edge of the forest.
Player 1: Okay, I ready my bow and start forward.
All other players: WAIT!!! We need to make a plan.
Player 2: Also, I'm still talking to this other guy
GM: Player 1, are you scouting ahead alone?
Player 1: No, I'll stick with the others until we have a plan.
All players: refuse to move while they spend 30 minutes arguing the
best tactics for hiking.
In this case you have mass analysis-paralysis. There are a few ways to break it:
Specifically tell the players that their precise plan doesn't matter. As soon as any encounter begins you will prompt them for their formation, and you will allow for sensible precautions. This doesn't suit all playing styles, but it saves a lot of time making preparations for things that never happen. The game style issue you want to resolve is, will the players be punished for acting without thorough explicit preparation? If so, then that might be why Player 3 is worried about bread, and it's the nature of a "10 foot pole to check for traps" game.
An extreme version of this is to run the game such that whatever the players suggest is considered reasonable. The world conforms to their expectations rather than the other way around. Then they don't need to plan at all, and the way to reach group agreement is not to wait until everyone agrees, it's to take turns to speak, and agree with and build on whatever the previous person said. They can improvise, they can take turns to contribute, and no matter how stupid what they do is, you will respond "yes, and...". Not "no, because" or "oh, this other thing first needs to happen first", or "you do that but you die because you never said you'd put your armour on". Again, this won't suit all styles, it's not very simulation-y. But they'll stop doing pointless boring things, because any interesting thing they think of is worth saying.
Guide the players through making the plan. You don't want to give them too much OOC information, but usually the characters have expertise that the players don't, and you can bolster their confidence by confirming their guesses and supplying general information at the right times. Confident people make decisions quicker:
GM: Okay, you're standing on the edge of the forest.
Player 1: Okay, I ready my bow and start forward.
Player 2: Hold on, I'm still talking to this other guy.
Player 3: Did we stock up on bread? I need to put on my night vision
goggles.
Player 4: [Crazy roleplaying thing!]
GM: Alright. Player 1, you're going to scout ahead, and you're ready
for trouble, that's sensible. Player 2, sorry, the other guy really
doesn't have anything else to say. Player 3, yes, you're fully
stocked, the night vision goggles will negate the darkness penalty
under the trees. Player 4, I like your style. Do you all fall into
formation behind player 1?
[In your transcript, at this point you said "are you guys going into the forest?". That is to say, you asked the group for a consensus before giving them any feedback on their individual issues. If you do that a lot, it's probably the main reason things get stalled.]
Player 2: One sec, I need to cast detect evil.
GM: I'm fine with that if Player 1 will wait?
Player 1: For one round? Sure.
Player 3: I'll stick close behind Player 1 since I have the best
sight.
GM: Good. 2, Results of Detect Evil are [whatever]. 4, once you're
done invoking the wrath of Gragnar on any fool who dares oppose you,
where are you in formation?
Player 4: Rear-guard, if everyone's happy with that. The Book of Gragnar commands us to, "Pity especially the fool who tries to sneak up you in a forest".
GM: Sounds good. Doing that thing you normally do when you're
rear-guard? That leaves player 2 in the middle. Onwards!
Design adventures so that the party doesn't have a lot of time to waste. You don't have to railroad, but make sure that there is always something happening to them. They can deal with it however they like, but they must act. After all, arguably if there's nothing happening to them and they're free to delay as much as they like then that's practically the definition of "downtime between sessions" ;-) So, players don't arrive at the edge of a forest at their own leisure, they arrive at the edge of a forest as a consequence of dealing with the previous problem:
GM: Okay, you're standing on the edge of the forest. You can hear those
enraged villagers with pitchforks approaching, but as you already know
they're very superstitious about the forest, and you suspect they
probably won't enter it this close to dark.
All players: start to plan
GM: [after a couple of minutes or so, representing the party's head start] The villagers have crested the hill behind you, and the front few break into a run. They're in missile range and will reach you in a minute or less, but then again you never thought much of their combat ability.
All players: No, we're not slaughtering the whole village! We get into the forest.
Finally, be aware that "keeping the game moving" doesn't need to mean physical activity or plot progression. If Player 4 goes off on a crazy roleplaying thing that the other players react to and enjoy, then it's irrelevant that it isn't part of your plan for the session. It's as much a part of the game as anything you invent. So encourage it to play out properly, and as long as it's not boring the forest can wait. Similarly, if the players just plain enjoy bickering in character over their plans, you can let that be part of what defines that particular campaign. Just build 20-30 minutes per significant group decision into your session plan. Less work for you!
To do this you need to get buy-in from the "more action-oriented players". If all they like is combat then that's pretty much a non-starter, you can't run a game this way for them. But otherwise you need to stop them tuning out by soliciting their responses, and making those responses matter in the conversation. A frustrated character who spends the whole argument saying, "we need to stop arguing about this and get into the forest" in 10 different ways is still an active player. One trick is to keep track of who is speaking, and if someone hasn't spoken for a while specifically ask them, "what do you think, what are you doing while this is going on?". That gives the player the freedom to take a turn in the conversation, or for that matter to wander off into trouble if they like. If you frequently find that you ask a player what they're doing and they say "nothing" and tune back out again, then you still have a problem and need to address personally with that player what they need from the game. Some players enjoy spectating for some of the time, and might look tuned out when they aren't, so you do need to ask.
Best Answer
You will have to improvise, to prevent de-railings.
When that fails, you may have to sit down and have a frank discussion with your party.
There are a lot of things that can be done to both allow for creative play and help keep a module on track. The longer you DM and the more practice you get the easier it will naturally become. So take a deep breath and dive in.
The first technique is to try and localize the impact of creative play. Your players are trying to make friends with the goblins so that they can gain some continuing tactical advantage. Localize the goblins, or in other worse, limit the scope of the goblins they do impact.
For example, you allow your party to make friends with some goblins. These goblins are part of a smaller tribe, and as soon as their defection becomes known, other goblin tribes are warned against defection and the goblins they did make friends with are now hunted. This could explain a hostility increase and refusal of future goblins to even speak with the party in encounters down the road as the pressure from other tribes / evil overlords make it very apparent that tribes seen talking to pink-skins will be dealt with harshly.
This could allow the party to gain a tactical or limited information advantage on a single encounter (like the known whereabouts of a lair, or the name of a bad guy) and then that information source is cut off. The localization also means that once the party leaves the immediate area of that small tribe, they would have to backtrack for their information source. Clans and tribes. D&D is not an industrialized homogeneous society. The feudal system is very modular and broken down into smaller squabbling factions held together by systematically larger and larger military powers.
But that's only one example of how to solve a specific problem. In my head I've divided adventure sourcebooks into two categories: Campaign sourcebooks and Module Packs. It is very easy to de-rail a Campaign sourcebook-style adventure, because of the open-ended nature of player choice of where to go and what do to. In these types of adventures, it's easy to go off-script. But there is a wealth of information about what else is going on in the world to help you recover or make a secondary plan.
The best course of action is to, rather than let your players run amok, allow the game to progress organically. Or, in other words: Let things play out as they would, rather than what your players want or how you think the adventure should go. You have time between adventures to play chess with the adventures with the other factions (and their responses) to things happening in the game. Danger, and the threat of danger, can do a lot to keep a player group tied down geographically.
If the Big Bad Evil guys hears about a new party of adventurers causing issues, what is to keep him from sending groups to counter and attack the support system of that party - to help wear them down? Or send a raid against the players' home-base town if they wander too far out of the area? Now the party will be more likely to stick closer to the designated area to protect the town. Or you can tie in the idea they need to do a favor for NPC X to help the villagers. This NPC just so happens to be the key NPC on the next module adventure, so it should work out.
But if the players don't buy your hooks or subtle nudges, you have only two choices: (a) Let them play out (Poor village gets nuked, and the monsters sent out 4 parties to raid the other 4 tombs simultaneously), or (b) stop the game and say honestly "Guys, were going to far off base here and I don't know how to handle it." I've had many DMs (myself included) do this, and the lengthy open discussion about where we want the game to go and kind of what needs to happen has done a lot to improve the quality of time at the table.
In situations like these, communication about the game the players want to play and the game the DM has prepared can go a long way to getting everyone on the same page. Sometimes letting the players in on a future basic plot summary can do a LOT to help keep players interested and playing along. Usually this is handled with foreshadowing, but that's not a skill every DM is good at. A little out-of-game knowledge can go a long way in this case.
The other thing that can come of this discussion is your campaign genuinely starts to move in a new direction. You just created a writers' circle for your campaign story. 5-6 heads are better than one, and now all the pressure you feel to solve this problem is being worked on by other people who care about the storyline.
Another thing that you can do is just say, "Hey, doing this will break my game at the moment and I don't know how to handle this. What can we do to marginalize this in a way that's acceptable, and then we can move forward with what I have prepared?" This is another common discussion I've had with other DMs. Good players should work with you on it. They care about the story too.