I once asked a producer how important casting is for a movie, genre or otherwise, and he told me "It's all you have."
It's always wonderful to see an actor who seems born to play a role, and it's also great to see an actor surprise you when they're playing against type.
Sure, we've all read stories about who was supposed to play a certain role, and how it would have made an enormous difference in the movie. Imagine Tom Selleck as Indiana Jones, or as Tim Burton's Batman, or Gene Hackman as Hannibal Lecter. In many cases, the alternate casting ideas would be interesting, and it's fun to play games with who could have played a role better, who would have been a disaster, and so on.
So the site Heyuguys.co.uk recently ran a story on perfectly cast films, and it got me thinking about what I would consider to be perfectly cast movies. In Jaws, you couldn't have picked three better guys to play Brody, Quint and Hooper than Roy Schieder, Robert Shaw and Richard Dreyfuss. In fact, Charlton Heston wanted to play Brody, and Spielberg turned him down, because you knew he'd beat the shark in the end from the first frame.
I also felt the Lord of the Rings series had perfect casting, although you do chuckle a little when you see Hugo Weaving, 'cause he immediately makes you think of The Matrix. It's also difficult to imagine who could have played the roles in the 1978 Superman better than who Richard Donner cast. Christopher Reeve was a wonderful star is born discovery, and Gene Hackman was a hell of a Lex Luthor. It also has what I feel was Marlon Brando's last good performance as Jor El. (Terence Stamp later had all of Superman II to shine brilliantly as General Zod).
And although the critics destroyed it when it first came out, I always felt that Popeye was a perfectly cast film. Originally Paramount wanted Dustin Hoffman and Gilda Radner to play Popeye and Olive Oil, but I don't think you could have picked any better than Robin Williams and Shelly Duvall. Great supporting cast too with Paul Smith, the sadistic guard from Midnight Express, as Bluto, Paul Dooley as Wimpy, and Ray Waltson as Popeye's pappy.
You could do a whole story on great against type casting too. People thought it was so revolutionary for Tarantino to cast John Travolta as a bad guy in Pulp Fiction, but Brian DePalma first cast him as a punk a-hole back in the 70's in Carrie.
Similarly, Michael Keaton as Batman is one of the most interesting against type choices I've ever seen (Jack Nicholson of course was the most obvious choice to play The Joker), and one of the best I've ever seen was Val Kilmer as Jim Morrison, a role many major actors would have killed for. I kept thinking back to the guy from Top Gun and thought, "That guy's gonna play Jim Morrison?," but he absolutely nailed it.
There's a lot that goes into a great movie for sure, and maybe casting isn't everything, but as the above-mentioned examples illustrate, it sure is damn important.