Solution 1: Harlequin
Have an extra player play NPC's. At start of scene, give them a card with their goals for the scene, and the NPC's sheet. If needed, give them also some information about what that character knows about the bigger picture. (But note that what they know may or may not be true.)
Solution 2: Expand the structure to 2+ Teams
Split the focus between team 1 and team 2, with players on each team. For example, if doing, say, any given police drama, each episode focuses on one team as the big, A-Story characters, with a smaller B-story by the other team. For Example, in Law and Order SVU, Bensen and Stabler are team A, and Tautuola and Munch are team B - in any given story, one of them is the lead story, getting about 2/3 of the episode, and the other team gets about 1/4 the episode. (minor overlap covers the remaining 1/12).
Solution 3: Narrow the Focus of the Characters
In Classic Trek, the "Trio" is Kirk, Spock and McCoy. Kirk is the action man and romance man, Spock the brains man and logic man, McCoy is the Heart Guy and medic. My last 4-player focused Bridge-Crew Trek game had the Captain who was Brains Man, psionics man and Social Sciences, the First Officer who was the Heart Guy and Action Man, the Doctor, who was both medic and logic guy, and the security chief and psionics guy. Note that the romance guy is absent, and the additional role still leaves some empty space for a 5th player. (Or that empty space can be used instead for some comedy of errors.)
Solution 4: Main or 2+ supports
Have players choose either to play 2-3 support characters, or one main character. Those who pick support characters get reduced points, and their two or three characters should be fairly different.
For example, if doing L&O Criminal Intent, Player 1 is Goran, Player 2 is Eams, Player 3 is the Captain and the Shrink, plus maybe Eams' boyfriend (in the show, not seen). A fourth player could be the Coroner and the ADA, and possibly Goran's mother.
TV in General: The Relationship Map.
The most important element in good TV emulation is a good clear map of the relationships.
Take, for example, Buffy. Who are the main characters? Well, for the series overall, Buffy, Giles and Xander. Why? Those three are in all seasons, and all other characters hang onto one or another of them. Monsters mostly attach to Buffy or Giles. In any given season, 1-3 others are 'main cast' - but the others vary by season. Anya, Tara, and Dawn are later seasons only. Willow goes away for most of a season, as does Giles. Giles doesn't get written out, tho - he guests every couple episodes - and his presence is still strongly felt even in his absence, and his prior relationships drive both a couple villains and the second tier characters. Willow's absence was fan-hurt, but not so much story loss, as most of her relationships overlap with Xander and/or Buffy.
Mapping the relationships out, however, reveals just how ensemble the plotting is for BTVS - Spike is linked to Angel, and Angel to both Xander (hostile) and Buffy (Romantic); Spike becomes linked to Giles, and later everyone. Anya starts linked to Cordelia, but then converts to Xander, and with Cordelia's absence, takes over Cordelia's role as Xander's love interest, and eventually Giles, as business partners.
Mapping out your party ahead of time allows them to come in with preestablished relationships, which makes the group dynamic much easier to maintain.
Round Robin with Troupe Style Rather than Group
Good start of game mapping can make it much easier to get a TV type feel. But also note that in TV-Land, not all the characters are in every scene. This can be emulated with what Marc Rein-Hagen dubbed "Troupe Style Play."
In Troupe Style play, every player has a main character, one that they alone play. The group also has a collection of secondary characters, which anyone not in the scene can play. These troupe-characters are included as needed, and usually, one or more are present in most scenes. They don't even have to be the same character to the same player. Focus on 1-2 of the main characters in the scene, and those not involved get to play a troupe character who could be in the scene.
Borrowing from Law & Order again, Dale Stucky could be a troup character, as could Dr. Wong, and any of the ADAs, plus a handful of recurrent uniforms, the Tech guy, and the Coroner. In such a game, Tautuola, Munch, Stabler and Bensen are all PC's. Only 1 or 2 scenes per "episode" would be all 4 PC's; all the others might be 2 pc's and 1-2 extras. Or 1 PC and 2-3 extras.
It's up to you
There are two play styles (with regard to this question at least), with their own drawbacks and advantages.
The first is to keep the realms of the world (DM) and the player characters strictly separate. I use the term "Golden Box" to describe this. In Golden Box gameplay, the players cannot define anything about the world, including who an NPC does or does not know - that is in the purview of the DM. Similarly, the DM cannot define background for the player character.
The advantages to this method include not having to deal with this sort of question. The player only knows the NPC if you, the DM, say they do. The player can argue for it, usually backed up with a background, but in the end it's up to you.
The disadvantage to this is that there is very little flexibility, and the players can feel like they have very little agency over the game.
The second playstyle, which I will not name since the only name I know it by lacks general context, allows anyone to define anything, and it is only addressed if there is an issue or will be an issue if left unchecked.
The advantage to this kind of play is that the players usually have a lot more buy-in - they have created parts of the world. In situations where this is taken to its logical end, the players define the world, the npcs, and to an extent the antagonists. This can be a very satisfying way to play.
The disadvantage of this is primarily it gives 'that guy' a free pass for a long time (to wreck your plot), and it can make a game feel directionless.
These are extremes. You usually mix them when playing a game.
I would recommend, in your situation, that you tend towards the first. When the player declares something about the setting that gives them an advantage, have them justify it, or if the situation allows, to roll for it. Don't be afraid to say No, especially if the declaration the player makes is problematic.
Best Answer
In my experience, actually hiding knowledge from all but one player is usually less dramatically interesting than the alternative. The reason is the concept of dramatic irony. Dramatic irony is when the audience of a story (in this case, you and the players) knows something that the characters do not. When you tell the whole table something that only one character knows, you now get to watch how that character explains the information in character, if they explain it at all. This is often hilarious, and just as often chilling. So most of the time, I think it's best just to tell the whole table and let them work it out in character.
That said, I think there are only two cases where you might want to pass notes, and they're rare enough that you can probably write the notes out before the session to save time:
1) When the character might have reason to keep the information to his/herself.
When Legolas sees riders in the distance, he's not going to not tell his friends about them. They're a team, and he's not a jerk. Hiding the information from other players and waiting for Legolas to convey it to them slows down the game and doesn't add anything meaningful to the experience. There are going to be very few situations where a character is going to want to hide information from the party. Mostly they come down to things like backstory, being secretly possessed by an evil wizard, or intra-party murder plots. If you're not running an evil campaign, this probably won't happen very often.
2) You don't trust your players not to meta-game.
This is the big one, and I think it's better to solve it by talking to players about roleplaying etiquette than by finding a way to pass them tons of secret notes. Make them understand that there is a difference between what they as players know, and what they as characters know. Even if they're A+ roleplayers, though, there will probably still be times when you want to avoid biasing their thinking. Maybe one character has been far away, scouting, and has some crucial information about an enemy attack, but you want the other characters to be standing around bickering with the emperor, completely oblivious, until the first character's breathless arrival.
Even in those cases, I find that a table full of good roleplayers is better able to help you tell a memorable story if the players have more information, rather than less. Basically, I'd say think very carefully about when a secret note would be helpful, write them all ahead of time, and stick to those instances only.