UPDATE: Cronenberg tells the Huffington Post that Rob was terrified to play Eric.. and that's a good thing:
Cronenberg: Well I'd watched a movie that I think not too many people have seen called "Little Ashes," where he plays Salvador Dali, and he plays him as a young man and plays him with a Spanish accent. So I thought, well that's really interesting, I mean this was before he was a "Twilight" star, because, it takes a particular handsome young man to decide to play that role. And then I did watch some of the "Twilight" stuff and I watched "Remember Me" and I felt that he had a lot going on. He's supposed to be a super smart billionaire at a young age, 28 he says in the movie. It's intuition. I didn't know him as a person, but I'd figured from the movies that I'd seen, like "Little Ashes," that I could maybe interest him in doing something that's not "Twilight" obviously.
Director David Cronenberg has been busy promoting A Dangerous Method, and lucky for us he's also been talking Rob. In this interview with Gothamist he talks about why Rob was the right guy for the film, and why Twilight played a big part in him getting cast.