His reasoning: "If you've met Joe, he's a really strong guy. I've never met an actor stronger than Russell Crowe." Liman goes on to say: "It's the Valerie Plame story, but you wouldn't know it when Joe's around. I met with [Russell] for Bourne Identity, and I was terrified. The whole time I was in the room with him I was just scared of him. This is a scary dude! And Joe is kind of like that."
Russell would definitely need to clean up and be aged a bit for the gig, but I guess he could pull it off. However, I wonder if Liman is focusing too much on the scary aspect. Strong and terrifying aren't really the same thing. But we're in a movie world where Josh Brolin is Dubya, so anything is possible.