RDJ has shown he can handle drama as well as comedy with Stark, which is all apart of his charm as Stark. Not the same as Dwaynes situation at all.
Like I said before, Dwayne is an actor. He's read off the script for the Rock and delivered those lines with charisma. Point blank. That's not the same as handling a dramatic role. About the 180 thing... I'm sure Dwayne has little in common with most, if not all, of the roles he has played. Still, none of those roles have indicated he can play a great Luthor.
See, there's the crux. An occasionally hotly debated one amongst film critics in what determines acting; presence of drama. I'm going to leave Dwayne here as this is bigger than him, but I don't agree the exclusion of dramatic elements diminishes a performance. It can be heavily argued (and it has) that evoking laughter is probably the hardest thing a performer can do.
Comedic timing is a trait which usually translates to smoother transition into the dramatic territory of acting. The pacing and delivery are meticulously coordinated at very specific moments to illicit a reaction from the audience. This is no different from when you're about to hit the punchline, or when you're about to unleash an enormous tug of the heartstrings. Is it any real surprise the actors who know how to control your emotions do so well in both comedy and drama?
Robin Williams - One Hour Photo
Bill Murray - Lost in Translation
Jamie Foxx - Ray
Steve Carell - Little Miss Sunshine
Jim Carrey - Eternal Sunshine of the Spotless Mind
Adam Sandler - Punch Drunk Love
Will Ferrell - Stranger than Fiction
Will Smith - Pursuit of Happyness
That's just off the top of my head, but hopefully the point is across. For whatever reason, comedy seems to have gotten a bad rep over the years. Truly an underrated artform.
Steering this back on track, it begs the question: would any of you guys be open to a proper comic actor taking on a dramatic turn as a famous comic book persona? I can only speak for myself, but the last time this happened (Keaton/Batman), I thought it turned out rather brilliantly.