Actually it probably should be two different guys, one to be the doctor and the other for when he changes into Thor. Also just how good of an actor do you have to be to play Thor? All you really need is a guy who's tall and really muscular to be convincing in the battle scenes, fantastic speeches won't be necessary.

t:. If he can't be superman then maybe he should send his CV for Thor? He need to beef up a bit with yellow hair a beard and voila!