Adopting a fake personality?
I do this aswell, though only if I've recently been exposed to (movie, television, book, etc.) a character that appeals to me. It's not a regular occurrence and when it happens I'm much more social but it's not really me and it's noticable for those who've known me for a long time.
I am guilty of this too. I have spent a lot of money on being like a certain person over the last year. Usually, I think it's a really odd, unhealthy thing to do. But if I think about it, it's good for building my own personality. I mean, doesn't everyone build up their self image from what they have seen of others?
I also use this for working out the correct social behavior to use in a social situation. This is how I manage to get on with people at work, but I have to watch as spending too long with them or interacting too long can give myself away. So no time is spent with them outside work. And a lot of walking away has to be implemented. I sometimes reflect on my behavior to other people and it can really worry me when I realise how much it varies from person to person and how unsubstantial my real personality must be. It's when different people from different situations all gather together at the same time; it gets messy.
Putting on a social mask is expected. You're not going to act the same way at a funeral as you would hanging out with your friends watching a movie. I have known people who have created whole personae for themselves. One guy pretended to be Cajun, another that he was from Argentina. The guy from "Argentina" called himself Ramon. When he dropped dead from a heart attack, many people learned for the first time he was actually Ray and was born and raised locally. He was a really nice guy. I would have liked him just as well without the intrigue. I can't help but think of the amount of concentration it would take to maintain that mask. It seems you'd have to be "on" all the time to avoid a slip up. Sounds exhausting to me. I've pretended to be interested when I'm not but I never have adopted someone elses personality. It feels like lying to me. I guess it depends on how much of a crutch it becomes.