It's hard enough to try to play somebody you are not. It's even harder when that character is trying to hide his personality and also pretends to be somebody else. And then, sometimes you are handed a character who can't even always control when the other person is in charge.
I just came from the larp where I had to do exactly that. Without going too much into the details and spoiling anything from those fellow players that might be reading this I was playing a character that was taken over by another entity but didn't have full control over the body. So obviously the original resident of the body surfaced every now and then trying to hinder the carefully maintained facade.
It was challenging, but at the same time extremely fun. Both in providing me with a nice challenge trying to do something hard and just being hilarious at the times when I had those Gollum like split personality moments and then, afterwards trying to explain to others what just happened trying to save the situation.
Even as I really tried to play to lose, letting out those slips generously it took other players quite a long time to finally figure out what was going on.