If it's convenient, I have the PC doing something else. But I prefer to not break simulation and have PCs "disappear" or otherwise slack off if they wouldn't. If the player knows they're not going to be there next session during the current session, it's usually easy to find an alternate occupation for their character.
Players usually have distinct personalities for their PCs that make it easy for the GM or other players to keep them on track; of course they don't get as much spotlight time as usual. As GM I'll run a character if I'm not juggling other NPCs; otherwise it gets farmed out to another player. (As GM I do chip in from time to time and reserve the right to intervene if I think a player sub is mishandling the PC badly.)
If a player has a specific request for that session they make it of the GM/player sub ahead of time; if I know there's a big decision point coming up I might ask them how they'd react (maybe with the serial numbers filed off). "Hey, if you found out your boss was holding out money on you and the other PCs wanted to kill him, would you help, not help, or stick up for the boss?"