Sure, a lot of TV shows are based on comics these days. But as much of the current genre material demonstrates, that leaves a lot of wiggle room for showrunners.

“Preacher,” which came to the end of its second season on AMC Sept. 11, is a case in point. It follows the tone, themes and broad narrative of the DC graphic novels, but veers widely from the specifics established by Garth Ennis and Steve Dillon in the comics.

Both comics and TV have the same premise: Two-fisted, hard-drinking preacher (and former criminal) Jesse Custer gains “The Word” — the ability to command people — and goes on a road trip with assassin/girlfriend Tulip O’Hare and Irish vampire Cassidy to find God, who has apparently abandoned his post.

So far, so good — the TV show follows that basic road map, at least in the second season. The first season, which acted as a prologue, was mostly invented for TV. However, the specifics in the two media are entirely different:

On TV, Eugene Root became disfigured in an attempt to stop a female friend from committing suicide and is accidentally sent to hell by Jesse. In the comics, he gains his disfigurement from trying to emulate Kurt Cobain, and doesn’t go to hell — he becomes an antagonist when Jesse gives his father a command that results in Sheriff Root’s death. On TV, Eugene escapes hell with his companion and guide, Adolf Hitler. In the comics — and this is a sentence I never knew I’d write — Hitler is not a supporting character.

And so on. Like “The Walking Dead,” another comic-inspired show on AMC, “Preacher” is faithful to the comic book in broad strokes, but invents its own secondary characters, digressions and plots. While the major events still occur, the details of the setup change.

Meanwhile, Fox’s “Gotham” returned to the air for its fourth season Sept. 21. The show is set before Batman debuts. If it showed fealty to the comics, that would be a fairly boring premise.

What we know about pre-Batman Gotham in the comics is that it had a lot of crime, but it was of an ordinary sort. Organized crime families such as the Bertinellis, Falcones and Maronis bedeviled the Gotham City Police Department.

In “Gotham” the TV show, the city is much more colorful place. We do get Carmine Falcone (John Doman) and a few other garden-variety gangsters, but we also get a hefty dose of supervillains who don’t appear in the comics until after Batman begins his crusade.

That includes Mad Hatter, Mr. Freeze, the Penguin, Poison Ivy, the Riddler and others.

“Gotham” also includes a pre-Commissioner Jim Gordon. Bruce Wayne becomes a vigilante this season as a teenager, which contradicts the comics, as well.

In short, there is no way to square “Gotham” with the various histories of Gotham and her citizens in the comics. It’s essentially a parallel world — which, luckily for Fox, is something that comics fans are very accustomed to.