Supporting characters who are very different from their players is a general and difficult challenge. It most often comes up in the context of social RP. We’ve got combat rules pretty much nailed down, in a lot of different systems, but there isn’t an equivalent that I’ve ever played where there are explicit round-by-round rules for social conflicts. There are definitely WAYS of dealing with social conflicts, and I have a MUSHing background, so I’m used to people taking a long time to come up with an answer that is ICly an instant reply. I’ve seen solutions where the same conflict system is used no matter what the stakes, and that’s pretty cool. But ultimately, a lot of games have step by step rules for combat, and no such rules for social conflicts. “I want to play a high-charisma person but I’m socially awkward’ is a problem that has various solutions, and not all of them are good.
Even fewer games (that I would want to play) have rules for what a character might think.
I often play the same sort of person: a bruiser with a brain. I enjoy being up in combat, and I enjoy the challenge of staying back from combat but still having meaning in an RPG which has a lot of combat rules. However, I find it very hard not to play someone who thinks a lot. My latest beloved character is of average intelligence, and it’s hard for me to play that, so I cheat.
She doesn’t think fast, but she does think a lot. Just slowly, sometimes. Behind the times. Or she’ll think things through but only part of the way. My main cheat, as this is a RuneQuest game, is to roll on INT. (That’s a Cthulhu Idea roll. Lots of system have equivalents.) If I don’t think of something vital at this point, I can think of it later, when it’s too late, but still interesting.
I also spam my GM with her IC thoughts. He deserves it. He got me into this in the first place.