Then, Conan asked Giamatti what role he wanted to play. Giammatti said, “I want to play one of the bad guys. What I hope they do is revive from my childhood, The Rhino as the bad guy.” We speculated when Paul Giammatti landed the role that the Conan appearance might have been the key in him getting the role.
Now, director Marc Webb has confirmed that was indeed the case. In an interview with Crave Online, Marc Webb said, “The real truth is, I saw Paul Giamatti on Conan O'Brien, talking about The Rhino and how much he loved it and I was like, I love Paul Giamatti and I think he's such a brilliant actor, so, I wanted to incorporate that enthusiasm that he had in the movie.”
So if Conan O’Brien had never asked Paul Giarmatti about his desire to have a role in the new Spider-Man movie, then it’s likely that Amazing Spider-Man 2 would have been Rhino-less. It's official! Paul Giamatti owes his Spider-Man job to Conan O’Brien!
Update: Taking things back one step further, it turns out that the source of the rumor that Conan O'Brien referred to was actually an interview that Paul Giamatti did with Collider, where Paul Giamatti was asked if there were any comic book villains that he would like to play. His answer seems to have started the wheels turning, which led to Conan's question and his eventual casting.