One of the more underrated roles in making a movie is casting. Many movies have been made because of a stellar performance given by an actor and many have become flops because of the lack of care or favors that had to be done in the casting process.
With comic book fans, especially before the comic book movie frenzy, they would debate who would be best in particular roles of their favorite heroes. Who would be best as Batman? Who would be the best Wonder Woman? So many opinions and yet, someone in Hollywood actually gets to play that role. Casting directors along with the movie director themselves get to play that almost fun role (although sometimes due to politics, might not be fun). Imagine if the Superman movie in the 1990's that was being led by Tim Burton, actually cast Nicholas Cage as Superman himself? A balding Superman with the personality of a door nail? No please.
Fans of novels get into the same debates, although not to the levels that comic books usually get. In the 2000's, Dan Brown's "Da Vinci Code" had such debate before Tom Hanks was chosen. Tom Hanks is a great actor and from a connections stand point, made sense as well since he has worked with director Ron Howard many times. Being a fan of the Robert Langdon character, I thought Johnny Depp would actually be a better Robert Langdon (he played a similar character in "The Ninth Gate"). The debate raged on recently as well for the "50 Shades of Grey" book series. Who would be the two stars brave enough to be open to play the kinky roles? Which guy would be the one to bring in the "mommy porn" lovers out of the home and into the theater?
Imagine how different "Godfather" would have been in Jack Nicholson played Michael Corleone instead of Al Pacino. Nicholson was offered the part first, but turned it down. How different would Don Vito Corleone was played by the movie studio's first two choices, Danny Thomas and Orson Wells? I bet it doesn't become the classic that it did.
On the vice versa side, imagine almost any movie Keanu Reeves played in. Now how much better is each one of those movies instead of Keanu Reeves? Keanu Reeves' only perfectly casted role was Ted from the "Bill And Ted's Excellent Adventure" movies. He wasn't even a top five choice for Neo in the "Matrix" movies. Will Smith, Tom Cruise, Kevin Costner (not kidding), Leonardo DiCaprio, Russell Crowe, Val Kilmer, and David Duchovny were all casting director's choices before they settled on Keanu Reeves. It's even rumored that before Hugh Jackman was cast as Wolverine, Keanu Reeves was the first choice. Horrible.
Movie casting doesn't get the recognition that a star actor, screenwriter, or director get, but it is one of the roles that can make or break a movie and should be recognized in a big, bad way, especially when it almost seems that a movie is perfectly cast.