Play Constructively: Pass the Ball is an excellent post on ars ludi that addresses players supporting fellow players at the gaming table. For example, if someone else is playing a spooky necromancer, passing the ball would involve having your PC always act like their PC is, in fact, a spooky necromancer — even if the other player isn’t selling it all that well.
I’d add one more item to the list, along the lines of number three: Push their buttons. Not to the level of being an irritant, but if another player has included angst, issues, phobias or other bad stuff on their character sheet, it’s because they want to see it come up in play. Help that happen.