I think that to have a realistic charry, you have to let them slip up once n a while, give them a few phobia's, a couple of allergies or illnesses (obviously mild ones ;D), let them fall on their face (physically of metaphorically) once in a while. I think its important to remember what you create about your character, so that yo don't find yourself RPing them doing something they wouldn't/couldn't do. Take my Character, Iris for example. She's terrified of water, in any large masses and in any form. She's alright as long as she's not near it (as in, one trip and your sat on your bum in a puddle). I've used this a number of times in RPs, one where she fell onto the icy lake during winter (was fun to RP her freak out, its worth doing ;D). So because of how huge I've made this phobia, it would be daft for Iris to decide she was going for a swim in the lake tomorrow. I've seen some people RP one thing, then completely contradict themselves a few RPs later. Its impossible to create a sturdy character if you don't set out everything about them. Think of your RP charry as a role in a play, and then use the Stanislavski Sytem (Method Acting). Create EVERY LITTLE detail about them, secrets, their childhood, even their family tree (I've done this one, and it's helped for some reason.)

So yeah ... thats my view on how to keep a realistic RP charry, or at least as realistic as it could be. hehe.