Just thought I'd share something I do in regard to role playing a variety of PC's. It's not unheard of around here to have Players playing anywhere from about 1 to 4 PC's at one time in a given game session. Whether that is all PC's or a combination of PC's and NPC's, it happens. (of course, if it's NPC's such as Henchmen, the DM (usually me) takes on more of an active role in determining things and the following is even more helpful I've found.
What I like to do when rolling up new characters is to use the NPC tables in the back of the DMG for NPC creation. These let you figure out the personality (somewhat) and dispostion of the character.
I like to think I am a pretty creative guy. I really like to give each character I create it's own unique back story and/or personality. I don't want every character to just be a shadow of every other character I have run.
Having said that, sometimes I don't have the luxury, time or ambition to go into all that, especially if I have just rolled up 2 or more characters at once.
So, I go to the back of the DMG and roll on the random tables to get a unique and random "personality" for the character. When things come up as to how the PC will react to something in game or what they might do in a certain situation and it is not obvious to me at that moment, I just look at the notes where I wrote all those down and it essentially tells me what the character would do.
I also keep a list of each monster that individual character has encountered as it goes along on the notes section of it sheet. That way, I know if that particular character has faced that particular monster or not and I can roleplay the character's action/reaction to it appropriately.
I do believe in the point of roleplay where the Player may be a 30 year veteran that has seen it all but the PC is a brand new PC and hasn't faced anything yet. Sometimes though, it's hard to separate one PC from another and that knowledge seems to get passed on anyway. Some Players give up and figure that it's easier to assume their PC does know what the player knows.
I prefer to keep things fresh per character. I think of myself as a spectator in those cases where I know what's coming but I can;t act like the character knows. Like watching a god old monster movie and yelling at the tv to the her o even though you've seen it a hundred times, the hero still isn't going to hear you yelling. heh heh.