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Whedon told me that he and Ruffalo both agreed that they wanted their version of Dr. Banner to follow the model established by Bill Bixby in the '70s TV version of "The Incredible Hulk," rather than how he had been portrayed in the more recent films.  The character on the show, they felt, "was busy helping other people.... That was more interesting to us than the Banner in the first two movies who was always fixated on curing himself."
Once the actor and director found a common vision for how they would handle Bruce Banner's personality, they had to work out the other side of the role: the lumbering, destructive, out-of-control Hulk.  Whedon admitted that he and Ruffalo fought over this.  Physically.  He said, "I mean literally we actually got some pads out and did some tussling.  Just to talk about the physicality of somebody who has to control this thing, and the way he moves in space and the way he relates to the people and the objects around him.  It was extremely fun."
Together, Whedon and Ruffalo worked out not just the dual nature of Banner's personality, but also the two sides to the Hulk's physical presence.  Whedon said, "What we found was that he could be very bumbling and kind of awkward, but at the same time very graceful and in this almost transcendent control of himself."  Their efforts should show on screen as well.  While the Hulk will be a digital creation like in the last two films, this time it will actually be Ruffalo playing the big green guy.  Using motion capture technology like in "Avatar," all of the Hulk's movements and expressions will come from Ruffalo.