I'm very aware of this at present as I'm running a solo campaign with twelve expedition members but the additional player. It's even harder in roleplaying games because it's one thing to act out one person faithfully, another to have whole conversations between yourself with two to three people which means that the screen time is generally a mere spotlight on one person at a time. It's also a shame because often we learn a lot of people by seeing how they interact with different types of people. We all wear different faces, at least slightly different, when dealing with others. The simple addition or subtraction of a single group member can change conversations and social interactions quite dramatically.
So what to do?
Well, I've sat down and picked out three traits for each of the twelve NPCs and then I turned those three traits into three lines and that's helped a bit. I killed off two NPCs, which means I only have to deal with eleven now, and their deaths have also given me a chance to flesh a few NPCs out through their reactions. Another one was reduced by 8 Constitution by moving plants (plants can be terrifying in Pathfinder, great horror fodder) and his wife has been trying to keep him alive although she had lost 5 Constitution. That man would have died if not for the PC providing him with a Cloak of Endure Elements and that gave a slight dimension to the two characters that would otherwise be in the background.
Still, Game of Thrones and, to a much lesser extent, the Walking Dead handle large casts reasonably well. How do they do that?
In a word: Politics.
Maybe you're groaning right now because you want an action-packed expedition with multiple members (like I'm doing) or you want to highlight a hamlet's population or survivors of a sinking ship and you don't want to resort to ... ugh ... politics. This isn't a political game, after all.
Umm, actually, it is.
The moment you get large groups of people in a room you get social politics. Their clashing and competing motivations and personalities will lead to a web of mixed feelings and considerations. The simple difference of 'who likes/dislikes who' can make as much of a difference to the group dynamics as creating a person who has a weak/strong temper. People are more likely to trust those they like and have mixed feelings if they do happen to trust someone they don't like very much (such as if they're good at their job). Also, when two people who don't like each other clash, it can add some extra spice to an otherwise boring scene.
That impatient and brittle character you created? Yeah, you'll reveal them more and in a dynamic and more memorable way when the brash character brings them close to tears. Even if you talk about the clash in third person rather than play acting it out, it'll *still* have a big impact. People naturally take note of social interplay and since it comes up more rarely in roleplaying games they're more likely to notice it, even if it's about as tangential to the plot as the blacksmith bitching to the player characters about how they can't trust anything they buy at the general store because the owner doesn't understand good workmanship (because he doesn't like the blacksmith's wares and said so once).
So yes, think about how the different characters relate. Pick any two characters and ask yourself what they think of each other. Create at least three options and pick the most interesting one. Consider reversing the connection or having their own feelings clash without realising it.
Billy thinks Tom is a brash idiot who'll get them all killed but Tom thinks Billy is his friend because he listens.
Tom thinks Billy hesitates and is too cowardly but Billy actually likes Tom because he gets things done and believes they complement each other's styles quite well.
Then while you're roleplaying, figure out how the characters might reveal their feelings. Even if they're trying not to reveal anything, they're bound to have noncommital responses where others might enthuse or they might visibly grit their teeth or roll their eyes when the other person speaks. Sometimes it's incredibly subtle and is in what they don't say and the facial expressions they don't make when talking about that other person.
- Social politics allows for dramatic moments between combats
- People can be revealed for who they are by what/who they like/dislike
- People can be revealed through how they respond to each other
- People can have different relationships with each other (one likes the other who dislikes them)
- People behave differently when around different people and that can round out the character
- Seeing interactions between other people can help the PCs get a better impression of them, but you don't need to act it out by swapping roles in an conversation with yourself. You can just summarise the encounter. That works as well.