When the movie 300 was first announced, those loyal to the graphic novel expressed concern that the movie wouldn't stay true to its graphic novel roots. After all, Frank Miller's 300 was noted for its unique artistic style: double-page spreads resulting in individual pages being twice as wide as normal comics, grungy color applications, and unique blood splatters.
This concern, of course was normal. Comic book movies such as Sin City (also by Frank Miller), Watchmen, Daredevil, The Punisher, etc., all had to confront worries from the comic book faithful. Some (such as Sin City) satisfy the original fans while others (Watchmen, The Punisher) fall short of expectations. However, in 300's case, the movie not only stayed true to the comic book, but seemingly brought it to life.
Miller has a pretty good track record of graphic novel to movie adaptations: His ever popular Sin City was met with rave reviews, while 300 sky-rocketed to the top of box office charts and home entertainment sales. Part of this can obviously be attributed to Frank Miller's involvement with both films: he played a role in addition to co-directing Sin City alongside Robert Rodriguez, and also served as producer on the set of 300. This allowed him to play a huge part in how the film was made, and allowed him to veto anything he felt was straying too far from his comic original. In Sin City, "much of the blood in the film also has a striking glow to it. The film was color-corrected digitally and, as in film noir tradition, treated for heightened contrast so as to more clearly separate blacks and whites. This was done not only to give a more film-noir look, but also to make it appear more like the original comic. This technique was used again on another Frank Miller adaptation, 300, which was shot on film."
In contrast, the film adaptation of The Watchmen was widely criticized by fans loyal to the comic. Despite being directed by the same director of 300, Zack Snyder, neither of the writers, Alan Moore or David Gibbons, had anything to do with the major production or direction of the movie. Eventually, Gibbons became an advisor for the film, but Moore refused to be associated with the film. Moore has stated he has no interest in seeing Snyder's adaptation; he told Entertainment Weekly in 2008, "There are things that we did with Watchmen that could only work in a comic, and were indeed designed to show off things that other media can't." While Moore believes that David Hayter's screenplay was "as close as I could imagine anyone getting to Watchmen," he asserted he did not intend to see the film if it were made."
Film adaptations of graphic novels are going to continue to be produced. However, the heavy involvement of the original author(s) can prove the difference between a film that pleases original fans or a disappointment for the comic book faithful.