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2. DON'T JUST CAST BIG NAME STARS
This should be a general rule for comic book and superhero movies, but one of the biggest problems with Hollywood is that it's still very star-driven and it's well believed that attaching a star to your big budget movie makes it more likely it will bring in audiences enough to be profitable. Sure, this is usually true for the likes of Will Smith, Tom Cruise, Tom Hanks, Brad Pitt and Angelina Jolie, but the key to selling a movie superhero is that people have to believe him or her as a character they've been reading in comics or seeing on cartoons for years.
The problem with casting Ryan Reynolds as Green Lantern was that it was casting done because he was a big name rather than because he was right for the role of Green Lantern, and it was hard for many to get past that after seeing Reynolds in all sorts of other movies and in the tabloids with his Black Widow wife, Scarlett Johansson.
Marvel Studios has done a good job mixing and matching, starting with the casting of Robert Downey Jr. as Tony Stark—Downey may be a star now, but let's not forget that he went to jail and was nearly persona non grata in Hollywood before Iron Man. They then went with a complete unknown like Chris Hemsworth to play Thor and had Samuel L. Jackson and Scarlett Johansson offer more star power for Iron Man 2, though in both cases, the casting fit the characters. Chris Evans had already established himself both as an actor and among comic fans before being cast as Captain America, and then they hired Oscar nominees whose prominence was growing in Jeremy Renner and Mark Ruffalo, both who helped make The Avengers one of the most popular superhero movies in years.
Warner Bros. is going the non-star route with the Man of Steel by casting Henry Cavill as Clark Kent/Superman, although that's also what Bryan Singer did with Superman Returns in casting Brandon Routh, who played the character for just one movie. If Cavill works as Superman, it may be easy enough to have a few better known actors playing the rest of the Justice League, and mixing and matching like with the Avengers, but the most important thing is to get actors that can lose themselves in the role and deliver characters that look and act like their comic book counterparts.