Talon:
Does anyone have experience with systems that helped to promote players to act in character? Whether in face to face roleplay or play-by-post format. If so, what mechanic was it that did so? (Aside from offering bonus XP or the equivalent for roleplaying!)
We should be clear on what each person means by roleplaying. Above, you say "act in character," and that's a good general description, but others have a more particular idea. For some people it specifically involves talking. For some it involves things that don't have anything to do with the challenges in the game; I've heard this called "playing house," not unkindly, by which they mean just living the characters' lives between tense situations. For some people, "roleplaying" is just about anything that isn't "combat."
My personal preference is to think of roleplaying as anything a player does that is plausibly what their character would do, whether that's talking, doing the dishes, hiding, throwing a punch or anything else. I prefer this definition, because it allows me to relax more about different playstyles.
Insofar as roleplaying is just doing things the character would do, many games promote it by making some things easy or automatic for a character and other things less so. In 4th Edition D&D, a fighter is well-equipped to go toe-to-toe with vicious enemies, thereby protecting their allies, whereas a wizard is less able to engage in melee but is well-equipped to stand back, zapping or blasting enemies, calling forth effects that non-magical characters can't easily reproduce, and applying personal protective effects when enemies get too close. In short, the fighter is encouraged to make fighter choices and the wizard is encouraged to make wizard choices. The two could swap places, but their options are more limited, so they're going to tend to be roleplayed as (the game's idea of) a fighter and a wizard.
One of the biggest things that promotes roleplaying is for players to feel like they can make in-character choices that won't a) be countermanded by the GM or other players or their characters or b) put them in a bad spot. This, I figure, is why combat and roleplaying are generally seen as distinct: the "good" choices are often extremely limited by the tactical situation, and doing anything else is likely to result in anger from one's teammates/fellow players or disappointment at one's own fate. Ever been in a game in which one person wants to talk to the enemy, or not go all out to kill them because that's "what the character would do" and everyone else is exasperated with them because they or someone else is going to get killed by this attempt at non-violence? (Or, a game in which someone attacks because that's "what the character would do" and everyone else is exasperated, because it's going to trash the negotiations?) It's like a chess player only ever moving their knights because their king likes to watch the horsies.
(Someone might make a character who tries to make only strategic, cold-blooded, calculating choices in combat and hates it when others don't. They'd then be roleplaying their character if that's how they acted during the game. If that's how the player is themselves, it's not much of a stretch, but I've never seen anyone claim roleplaying requires one to stretch.)
Along the same lines, if player ideas are deemed not to be possible simply by the rules of the game, a player might feel that they can't "be" the character they had in mind. If the rules (either as written or in the interpretation of the GM) make it very difficult or outright impossible to swing into or out of a situation on a chandelier in a game of swashbuckling adventure, then it might be hard for some players to feel like it's worth trying to pretend to be a swashbuckler.
And so, all that said, I do feel like Fate does as well as any came can in encouraging playing one's character and not being afraid of their own or anyone else's sub-optimal choices. In theory, actual "optimal" play
requires a few sub-optimal choices. At the same time, players and GMs are encouraged to allow player ideas to work, and gives everyone currency to essentially pay to get their way, rather than having to argue or justify their point. This makes it possible (or at least more likely) for everyone to have the conditions they need in the game to feel okay about doing what their character would do, instead of some supposed "right" thing.
I'll also mention Dungeon World. Dungeon World gives a player experience points if they fail a roll. This goes some way toward justifying a player making choices that are more about what the character would do than about success. Failure also brings about something that's probably not in the characters' interests, so there's not much upside to doing lots and lots of things the character is bad at, just to gain experience.
Dungeon World characters also gain experience for doing something related to their alignment, which works because each character sheet clearly explains what that something looks like or results in. I wouldn't try something like that with, say, the D&D alignment system.