Players want play time. Forget about treasure, XP, or hero points: the only reward that really counts is getting to play. Would you sit out a game to get more XP? Would you play for half as long if you could get twice as much treasure? Maybe if some trade of less game now = better game later. Otherwise, not likely.
In your game there are X players, and since everyone really shouldn’t talk at once, each player is getting about 1/Xth of the playtime. Some players get more, some get less, but basically the more players there are the less time each player is going to get to actually do things in-game.
A ha, you say, that means that giving a particular player more play time is a reward! Basically yes. A player who gets more time in the spotlight is getting to play more. It’s warm and cozy in the spotlight. But it can also get hot.
Let’s say you create a game that really focuses on a particular character. Fred’s playing a priest and you plan a whole session with the party visiting the church fathers, dealing with internal church politics, and so on. All priest politics, all day long. Even if the other players are into the plot and love the idea, it means a lot of play time for Fred. He will be involved in a lot of the scenes, because he’s the party’s link to the action. He should be happy, right?
The problem is that the game hinges on Fred. If he doesn’t play his priest well and provide a good window into the situation, the game falls apart. The other players are sitting around waiting for Fred to talk to people and move the plot. That’s a lot of pressure on one player.
Every game session players get to choose how involved they want to be. If they are in the mood, they can leap in and roleplay. If they are feeling mellow, they can usually sit back and let the party carry them along. People have good days, and people have bad days, and sometimes folks are just too tired or in the wrong mood to make a big effort.
The spotlight character doesn’t get that choice. The game stops with everyone’s attention square on that lucky player (“your wife confesses she’s having an affair with Dracula! What do you say?”). If he doesn’t step up, the game grinds to a halt, or at least limps along.
Sometimes the spotlight turns on even when you didn’t expect it. The game might not be specifically about one character (“Look, it’s Fred’s long-lost brother!”) but it might still be a situation that clearly calls on one character to step up and roleplay. When you bump into a bunch of xenophobic elves, the elf in the party is probably the one who has to do the talking. If one character is the self-styled Egyptian scholar, they’re naturally the party’s “what’s up with that mummy?” resource.
Yes these are situations the player should be welcoming, since they are situations that (probably) give the character a chance to showcase the traits the player choose, the traits the player wants to play. But maybe not today.
So what do you do? Be aware of when you are setting up spotlight situations and putting the burden of the game on one player. Always be ready to give a player an out if you have to. You can challenge your players to roleplay, but in the end you have to let them choose the level of their involvement in any game session.
Yes, it follows that giving a player less play time is a punishment. Invite someone to the game and then have them sit for four hours waiting to join in. Do you think your game is so interesting that the player is happy just to sit and listen? Think again.