Relevant to this topic: Keith Johnstone's 'Masks'. It would be better to read the relevant section in his book "Impro" for the whole story (I got it at my university library) but this collection of quotes followed by this video should give enough of an introduction.

The idea is that while the people wear these masks, they are able to become a character with a personality different from the actor's original. The actor doesn't feel as if they are controlling the character. That being said, it doesn't happen immediately: It can take a few sessions for the actor to get the feel for the thing. The other thing is that the Masks usually have to learn to talk (albeit at an advanced pace) eventually taking on the vocabulary of their host. It's very interesting reading, to say the least.