(pic from TheCinemaSource.com)
Exactly what happened to Christian Bale's accent during his famous rant now buzzing around the interwebs? Listen to this 2002 interview with the "Welsh born" actor, then listen to the famous rant here. In the 2002 interview, he quite clearly has a UK accent (though it's not clear to me exactly what variety since I'm not good at placing UK accents; also, he spent relatively little time in Wales, so knowing his birthplace is not much help). Then, in the rant, he has largely what I would call a American English/California accent, but it breaks occasionally (into what, I'm not sure). It might be the case that Bale's famed intensity as an actor engulfed him so much that he was still "in character" when ranting, I don't know.
I never studied the factors of accent change, but it seems like an interesting topic. According to Bale's Wikipedia page, he moved around a lot as a kid, and according to Bale himself in the 2002 interview above, he still moves around a lot as an adult, so he moves between many speech communities.
I'd be curious to know how phonologists have modeled accents. Is a person's accent simply the accumulated total of word pronunciation norms, or do we have a model of a holistic accent in our head that we are trying to approximate when pronouncing sentences (an accent grammar, if you will)? This seems like a non-trivial distinction.