I read every installment of Daniel Clowes’ graphic novella Ghost World when it was originally serialized in Clowes’ comic book Eightball back in the mid-1990s, so when I heard Clowes had collaborated with Crumb director Terry Zwigoff on a movie version of Ghost World—and read some of the early raves for the film—it became my most anticipated film of 2001. Ultimately though, I found the Ghost World movie disappointing. The film’s bright colors—and its emphasis on one middle-aged man’s grumbling—struck me as too off-model from the muted, female-focused coming-of-age story I loved in comic-book form. As the year played out, and more critics talked up Ghost World, I bit my tongue, not wanting to be one of those insufferable grumps who goes on and on about how a movie failed its source material. Yet even today, I think the Ghost World comic is beautiful, and the movie is scattershot. And the reason for that is that comics and movies aren’t as closely aligned as some may think.

Because comic books look a lot like motion-picture storyboards, comics seem like they should be easy to adapt: Just position the camera with the same perspective as the comic-book panel, have the actors recite the dialogue verbatim, and then no one can say the filmmaker didn’t faithfully translate the comic to the big screen. But while cinema and comics are both strongly visual mediums, they follow different conventions. Superhero comics in particular often depict action and dialogue happening simultaneously, in ways that would be impossible to film. And comics of all kinds often vary perspectives from panel to panel, in ways that would look choppy and visually incoherent if a movie did the same. The art of adapting comics often involves taking specific memorable images from the original pages and then filling in the action so it’s all more cinematic—similar to how the cel animators of old would draw the key points of motion in a scene and leave the rest of the movements to the “inbetweeners.”

Frank Miller and Robert Rodriguez’s Sin City movies offer a counter-example—one that exposes the limitations of staying faithful. Miller and Rodriguez use digital technology to copy the look of Miller’s original artwork fairly precisely, right down to his high-contrast lighting effects. The Sin City movies also try to replicate the comics’ evocation of old dime novels and pulp magazines, by treating each individual film as an anthology of interlocking stories.

I haven’t seen Sin City: A Dame To Kill For (which opens today), but while I find Sin City interesting as an experiment, I don’t think it’s all that successful as a film. The Sin City movie reminds me a lot of “motion comics”— that bastard form of straight-to-video entertainment that adapts fan-favorite comics storylines by adding limited animation to the original artwork. To me, Miller’s drawings are more dynamic on the page than they have been in live-action, where they’ve been so weighed down with makeup and effects that they barely move.

That said, Sin City is truer to its creator’s actual writing than a lot of comics adaptations ever attempt to be. That matters, because Miller’s Sin City comics aren’t just about a batch of recurring characters and an organizing premise, as a lot of superhero comics are. Miller gives the dialogue and plotting in his Sin City comics a precise punch. Too much divergence from that, and the film would cease to be Sin City.

My main beef with Ghost World isn’t that I wanted the movie to be a panel-for-panel copy of the comic; it’s that Clowes’ conception of his two above-it-all teen-girl protagonists is evident in the film only fleetingly. Even though Clowes was involved in the production—and even though Zwigoff had longstanding connections to the alt-comics realm as a buddy of Robert Crumb—the Ghost World movie doesn’t trust that Clowes’ original elliptical, episodic storytelling is strong enough to support a movie. The comic itself recalls lightly bruised youth movies like The World Of Henry Orient and Harold & Maude, but the film is more like a compendium of funny bits from various Clowes comics, periodically filtered through the perspective of a grumpy older man, played by Steve Buscemi. The movie Ghost World has its merits, don’t get me wrong. But to me, it’s not Ghost World.

There’s an old saying that great books don’t make great movies. That doesn’t exactly hold true, because multiple masterpieces of literature have been adapted into excellent films. But while there have been great movies adapted from comics, I can’t think of too many great movies adapted from great comics. Or, to be more exact: It’s hard to come up with too many top-shelf pieces of cinema that hew closely to an essential graphic novel, or to one memorable comic-book storyline. (Persepolis probably comes closest.)

Consider the fate of writer Alan Moore, whose books From Hell, V For Vendetta, The League Of Extraordinary Gentlemen, and Watchmen have been screwed up on the big screen every which way: Some of the adaptations have been way too loose, while others have been so faithful that they’re choked off. One big reason for that is that Moore’s voice as a writer is too distinctive. That makes it harder for screenwriters and directors to do their job, to be respectful to Moore’s work while putting some kind of original, personal stamp on it. This isn’t an impossible assignment. American Splendor takes decades of Harvey Pekar’s autobiographical comics and reassembles some of their best stories into a movie that’s partly a Pekar anthology and partly a commentary on what Pekar represents. If only someone could apply that kind of imagination to a movie version of Moore’s Tom Strong or Supreme comics.

People who adapt long-running superhero comics have their own issues to sweat, due to the decades of interwoven storylines, retcons, reboots, re-imaginings, and fan-favorite incarnations. From the mid-1970s to the early 2000s, filmmakers got around all that by ignoring it, figuring (not entirely wrongly) that comics readers would be so thrilled to see their favorite characters on a movie or TV screen that writers and directors could take whatever elements they liked from the comics and then add their own interpretation. After the success of the first X-Men and Spider-Man movies, though, “true to the comics” has become more of a goal—and there are plenty of superhero fanatics on the Internet ready to cry foul if a movie doesn’t hit that mark.

The advantage superhero movies do have is that there’s rarely one lone voice that the filmmakers have to follow. Even Captain America: The Winter Soldier only adheres to the roughest outline of Ed Brubaker’s comics storyline of the same name. By and large, superheroes—or at least the superheroes published by Marvel and DC—are considered malleable properties, which new creators can mold into their own visions. Comics fans have grown used to someone with new ideas (like Ed Brubaker, for example) coming in and doing some redecorating.

Which brings us to Guardians Of The Galaxy, which when all of 2014’s receipts are counted stands a good chance of being the highest-grossing comic-book movie of the year, unless the nit-picky want to claim Transformers: Age Of Extinction as a “comic-book movie.” Together and alone, the characters in Guardians Of The Galaxy have years of adventures in print, some of which have undoubtedly informed the movie that writer-director James Gunn and co-writer Nicole Perlman released this summer. But there was no one comic-book collection on the shelf pre-Guardians that told anything like the story in the movie. And that had to be freeing for those behind the film: to have no standard that they might fail to uphold.