According to Joseph Gordon-Levitt, when he prepares for a role, he often takes on aspects of his character in real life.

So for "The Lookout", where he played a janitor who plans a heist, he took on custodial work at a local bank...

And for his most recent role, as Cobra Commander in "G.I. Joe", he integrated himself into a militant organization, slowly rising to the top to obtain the rank of...

Or for "Mysterious Skin", where he played a gay prostitute, he...

Come to think of it, his role on "3rd Rock from the Sun" as a Jewish alien wasn't that much of a stretch.