“Casting Captain America is really casting two roles… Steve Rogers before and after the transformation from 98 pound weakling to perfect physical specimen. I can’t divulge how we’re going to do it, but the performance will be Chris Evans from beginning to end,” Johnston said.
In looking for the perfect actor for the role, Johnston added that the “actor playing Captain America has to embody that backstory and let it inform his understanding of his new self. One of the interesting aspects of the character is the effect that kind of total transformation would have on the his psyche. It gives the character more dimension and the actor playing him a wider range of interesting choices.”
You can read more of Johnston’s quotes here.