Lee Child's character "Jack Reacher" is described as 6'5" and about 250 lbs. while Tom Cruise is much closer to 5'6" and a buck-fifty.
That is the Hollywood Effect. Producers will get a star who will get seats filled in the theaters, not necessarily that fit the original description of the character. Yes Tom Cruise is a midget compared to what Jack Reacher should look like but finding someone who can act that is that size would be difficult and people might not want to pay to see an unknown in the lead role.
John Constantine was first appeared in an issue of "Swamp Thing," created by legendary comic book writer Alan Moore. Several years later the character would garner his own comic book series "Hellblazer," written by Jamie Delano. It has proven to be quite popular as it is the longest running title under DC Comics' edgier venue, Vertigo.
When Francis Lawrence began working on his first feature film, 2005's Constantine he knew he was going to hear an earful from fans as he stripped away the character's blonde locks, British accent and switched the setting from London to Los Angeles. But hey he still had lung cancer and a trenchcoat.
Having Keanu Reeves fill the role of the renegade occultist certainly didn't befriend Lawrence to the fanboys either as the actor doesn't look anything like the singer, Sting, which the character is meant to resemble, nor has the actor's talents ever been confused with Sir Lawrence Olivier. But outside of diehard fans of the comic, most were able to overlook those superficial changes, and except Keanu in the role as Lawrence did an admirable job of conveying the character's darker qualities. His Constantine was still the chain smoking, self-destructive, cynical man with a surly attitude. Although I would say Lawrence's biggest achievement with the film was the overall gloomy look and the amount of meaningful violence that he was able to include in a PG-13 rated film.
On the other hand, Buscemi is not physically imposing. The actor has a high-pitched voice, stands just 5 feet, 8 inches tall, weighs under 150 pounds and is very slight. He never wears glasses, even when portraying Nucky."