Ruffalo's performance as Hulk was praised upon the film's release, as was Joss Whedon's writing of the character. What might have gone unnoticed at the time, however, is that this was the first portrayal of Big Green to use motion capture. Both Eric Bana ("you won't like me when I'm Ang Lee") and Edward Norton's Hulks were both constructed entirely from CGI.
The video below shows how they were able to do this. And here's a spoiler: there was no Andy Serkis, nor Lou Ferringo covered in green body paint.
Enjoy...