Mark Ruffalo is a terrific actor and always fun to see in movies, but he isn't my idea of Banner.
I visualize the guy as, I dunno, a cross between Christopher Gorham and Tom Hanks.
If Ruffalo wants a Marvel character role, I'd give him Dr. Strange instead. He'd be able to play offbeat, playful and menacing all at the same time.

If Ruffalo wants a Marvel character role, I'd give him Dr. Strange instead. He'd be able to play offbeat, playful and menacing all at the same time.