It's a joy to see an actor who once seemed nothing more than a bimbo, a pretty face, mature into a real actor of far greater range.
These transformations are electrifying: Think of Brad Pitt as a mental patient in "12 Monkeys," or Johnny Depp circa his "Arizona Dream"/"Ed Wood"/"What's Eating Gilbert Grape" period in the early 1990s.
No less satisfying is seeing a long-term character-actor morph into a leading man. Think Jean Reno in "Leon," Paul Giamatti in "Sideways" or Philip Seymour Hoffman in "Capote" and "Charlie Wilson's War." Give them a bite at the lead, and they prove more than capable of carrying a film.