It's no secret that comic book adaptations have been dominating the box office lately, in the United States and internationally.

The film industry has always looked to other art forms for source material, whether it be novels or stage plays. Comic book movies were once a major risk, considering the large budgets typically involved and the perception of comics as "kid's stuff."

Thanks to major technological advances, movies have only recently been able to capture the spectacle and imagination of superhero comics. Comic books proved to be reliable source material for blockbusters throughout the 2000s, culminating in the massive successes of The Dark Knight and Iron Man in 2008.

The link between comics and film may have only recently become obvious, but the two medias have shared a great deal since the early 20th century. Where most forms of storytelling are driven by written or spoken word, comics and film (and, more recently, video games) are unique in their visual focus. The two media are at their best when dialogue and narration are used to support the narrative, not to tell it. The mantra "show, don't tell" may have originated in literature, but it became one of the foundational principles of screenwriting.

Due to their common elements, movies and comics have influenced one another quite deeply. However, the two media are still extremely different, and stories often make major adjustments in the adaptation process. While readers have long complained that "the book was better," it's even more uncommon for comic book adaptations to stick to the source.

No comics creator has seen their work adapted as faithfully as Frank Miller, the writer and artist behind 300 and Sin City. The adaptations of both stories are visually and narratively close to the source, borrowing dialogue verbatim and recreating his artwork with obsessive precision. This is in large part due to his uncommon level of involvement in the adaptations, having served as co-director of Sin City and executive producer of 300. Both adaptations were also helmed by directors (Robert Rodriguez and Zack Snyder) who are known for extremely stylized films.

These stories were also particularly well-suited for film adaptations. 300 was a simple standalone story that fit within the constraints of a two hour film, free of the complex continuity that characterizes mainstream superhero comics. Sin City was inspired by film noir, featuring the visual style and stock characters commonly used by the genre. Few comics lend themselves so well to film adaptations.

Superhero comics mainly unfold as self-contained issues or long story arcs, often featuring several issues and countless tie-ins. Single issues are far too short to provide the basis for a feature film, whereas full story arcs are usually too long and complex. As a serialized medium, superhero comics may draw upon decades of previous stories, and assume the reader has some level of familiarity with them. As a result, most comic adaptations mix and match a few minor elements from existing story arcs, but follow a mostly original storyline.

Marvel can publish hundreds of comics in a single year, but only produce two or three films in the same time frame. Attempting to capture the vastness and complexity of superhero universes can result in movies feeling overstuffed and convoluted. Last year's Avengers: Age of Ultron struggled to balance its massive cast of characters, and even this year's Captain America: Civil War was generally agreed to be another Avengers movie, with far too many characters to really be considered a Captain America story.

Obviously, Hollywood's focus on superhero adaptations is playing out quite successfully. However, it's worth noting that superhero comics do not naturally lend themselves to film adaptations, while comics from many other genres do. This isn't to say that superhero adaptations are a lost cause, but that comics have a lot more to offer than superheroes.