How do you handle being absent a session when you have specific actions in mind for your character?
If I am not there, in general I do not want someone else playing my character. If there is no logical way for your character to not be there, our group normally just assumes the character is there giving general input that is not essential to the current events or is helping "off screen" in combat. This is not to say I don't mind my friends saying my character makes a hilarious comment due to his personality or brings up a good point that my character WOULD know if I was really there that the others don't, but in general my character is left out of the heavily character involved things if not absent completely. It's hard to make assumptions about how someone or a character would react even when you think you know the person/character well. This is the same reason why I do not like playing already established characters in any setting (e.g. like playing Darth Vader in a Star Wars RPG game).
When GMing I have characters of absent players fade into the background. I will only use their capabilities or have them speak up if they would have really useful information I am certain would be divulged, I think I've only ever done this once, however. The characters do not engage in combat at all if the player is absent.
As a player I expect my character to fade into the background too unless the GM feels they really need to bring it to the forefront, as above.
Player not there = character not there for the most part. Not exactly realistic I know, but then again this is all make believe ;)
I talk to the GM about it ahead of time. The GM is happy because he has a heads up about the absence, and he can often provide immediate feedback about how feasible what I want to do is (and whether or not he'll allow it for an absent player).
Trust is key, right? I like to be surprised! But if there is something I really, really want to see happen, and we can't arrange for the in-game events to accommodate my schedule, I just lay it out and ask my friends - GM and players - to do their best.
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?"
In our group, we usually do one of a couple things. Either you pass your stats to another player to play for you, or the character is run by the DM as a background NPC for the session and the DM only involves the character if the party absolutely needs them. Occasionally, though, the missing person's PC become incapacitated somehow or teleports/goes away to conduct business for the evening.
As the GM my rule was always that PCs of players who failed to show were my NPCs for the session and I would play them conservatively with the express intent to keep them alive if possible while retaining at least a little of their personality's flavour. They also didn't get as much XP (in the games where this was an issue) for this -- no risk, no gain. The reason for this policy was that I tended not to use the picaresque style that most RPG campaigns work off of so there was no way to have characters suddenly "off doing something else" for an adventure -- there were no adventures, per se.
As a player my policy was to follow whatever group consensus was. Typically that consensus was either like my policy as GM (since I tended to game with people who shared my tastes, oddly enough) or, for the more picaresque-style campaigns, my character was off somewhere else like "visiting the old homestead" or something along those lines.
In my group, we have myself and 2 couples. If everyone gets together, we play the main game. If one of the two couples can't play, then we play a side game (where that couple or myself does not have characters). Typically, we play many different games, and not all settings will appeal to all the players. If, for example, I hate Dark Ages Vampire (which I did, Characters never got along at all), and I can't make it to game, the word gets out to bring Dark Ages stuff, and usually the night is spent getting Dark Ages up to speed and moving the plot forward a bit (not a full session's worth, as the Storyteller didn't have much time to prepare stuff, and we are old enough that memory is beginning to fail us). We continue with the main game the next evening. Won't work for everyone, but it works wonderfully for my group.
