Fans of Stephen King's The Dark Tower are some of the most dedicated around, and some of them are nervous about director Nik Arcel's upcoming big screen adaptation. Arcel understands that feeling perfectly.

"If I hadn't directed this film, I would be on online right now going, 'Where's Eddie and Susannah? What the hell's going on?!'" Arcel told GameSpot during San Diego Comic-Con this week. "Like, I would be so nervous, really. So I know exactly how they're feeling. But that's why I feel comfortable, in a way--because I know that we are true in our hearts, and that we actually did everything we could."

Look no further than the director's recent ask-me-anything session on Reddit to witness fans' fervor over these questions. Eddie and Susannah are absolutely crucial characters in the grand scheme of the series, and fans are concerned by the fact that they--and countless more elements of the franchise's mythology and world--didn't make it into this film.

As Arcel pointed out, they weren't in the series' first book, The Gunslinger, either. Written when Stephen King was just 19 years old, the first Dark Tower book is short, surreal, and laser-focused on three characters: The gunslinger (played by Idris Elba), the man in black (Matthew McConaughey), and the boy, Jake (Tom Taylor). So, too, is the movie, which Arcel believes makes it faithful to the series.

"I always felt like you can't possibly adapt this. You can't adapt The Dark Tower. It's too dense, it's too sprawling, it's too many different genres. Every book is a different beast," he said. "But then when I read the script, I was like oh, that's really smart. It actually is tight, it's lean, it serves as kind of an introduction to the world, the characters. It's really a first film. It's a first film, but it still takes little ideas from some of the other books, and so makes the world feel more rounded in a way."

"And the script was lean, you know?" he added. "It wasn't like we cut it down or anything. It's pretty much the script.

There's no guarantee that Arcel will get to make more Dark Tower movies after this one, but he's prepared for the possibility. And he said future Dark Tower films could potentially stick closer to the books, once this initial film introduces audiences to the story.

"It can't be a straight up adaptation, but it can certainly become one the further we get into the series," he said. "I mean, let's say we get to make more movies, it can actually be more and more faithful as we go, because what we had to do on this first one was basically just like: This is the world. These are the characters. This is what this world's about."

The Dark Tower's fans are incredibly passionate, but they're also divided. Some prefer the earlier books in the series, others the latter. Some love the fact that Stephen King wrote himself into the story as a character, and others despise it. (Arcel said King told him never to do that: "He was just like, 'Just don't ever put me in any of the films. No character called Stephen King in there.'")

One thing Arcel knew had to change from the first book is the ending, and there's an exceedingly simple reason why: Narrative payoff.

"The first book was lean and tight and was kind of simple, and the first book was all about Roland, Walter, and Jake. And so is this film: It's Roland, Walter, and Jake. Eddie and Susannah, they come later. Oy comes later. So we're actually being kind of true to that," he said. "But obviously, there's some things in the first book that we couldn't do. I mean, we couldn't have Roland pursue the main villain the entire film for them to just sit down and have a little chat at the end, like, 'Hey, man.' So that's why we've gotta have them have it out at the end, instead of just sitting and talking by a campfire."

"I wouldn't want to ever do a film that ended on some sort of cliffhanger, you know, 'and here's what's going to happen if you go see this one!'" he continued. "I think it's very important for any first film in any franchise, or in any kind of saga that you're trying to tell, is of course it has to work as a standalone film. Because you never know, and you also want to give people a satisfying film, right? There has to be some sort of conclusion."

The Dark Tower hits theaters August 4.