He’s back (as if he ever left).

On Oct. 4, “Joker,” starring Joaquin Phoenix as the titular lunatic, comes to theaters. Directed by Todd “The Hangover” Phillips, the movie has already stirred up all sorts of noise.

Yes, it won best film at the Venice Film Festival, but a U.S. Army office also already received information about a “credible potential mass shooting” tied to the release. Yes, Phoenix’s performance is supposed to be extremely extreme, but reviews out of Venice have led to the creation of at least one meme putting the word “8Chan” on the poster where “Joker” should be. (An online message board, 8Chan notoriously has served as a platform for extremists and hate speech.)

We can, perhaps, agree on one thing: This weird fictional fellow has occupied our imaginations forever, a green, white and purple symbol of our id at its worst. Let’s take a quick look about where the Joker’s been, where he’s going and why he just keeps hanging around.

The Joker first showed up in the comic book Batman No. 1, back in 1940. He was created by some combination of writer Bill Finger and artists Jerry Robinson and Bob Kane — nobody has ever agreed. It seems oddly fitting that a character whose true backstory has never been revealed in the comics should have his real-life beginnings be the subject of so much bickering, credit-theft claims and fuzzy memories.

We do know, famously, that the Joker’s hideous grin was based on actor Conrad Veidt from the 1928 horror flick “The Man Who Laughs.” It’s a world-historical character design; comic book creators have been trying to live up to that nightmarish visage ever since.

Related: Wilco, ‘Joker,’ and a Jack Reacher novel: A guide to pop culture in October

At first, the Joker was a straight-up murderous psychopath who seemed to kill as a first rather than last resort. He was fond of elaborate assassinations of the rich and secure (not out of some sense of justice, mind you, but because that’s where the money was). While the Batman wants to strike fear into the hearts of criminals, the Joker just wants to strike fear.

In the mid-1940s and into the 1950s, the Joker became more cartoonish, and his schemes became more elaborate.

He even got an origin in 1951′s Detective Comics No. 168, in which it was revealed he was low-level hood in, well, a red hood who called himself (wait for it) the Red Hood. After being chased by Batman, he fell into a vat of chemicals which turned his skin white and hair green. Mind you, we still didn’t really know anything deeper about him, which made him all the scarier. This essentially is what happened in the 1989 “Batman” movie starring Jack Nicholson as the Joker, with the added and unfortunate implication that a younger, pre-chemical Joker killed Batman’s parents. Talk about missing the point of Batman having his origin in a random crime — in that flick, it plays like destiny.

In the 1960s, both Batman and the Clown Prince of Crime made huge comebacks thanks to the “Batman” TV show. Cesar Romero and his white greasepaint-covered mustache played Joker as a cackling fan of death traps. But here is the thing: He seemed no more nuts or dangerous than any other Batman baddie on the program. And anyone who dislikes this series but sits down every week for “The Flash” or especially “Legends of Tomorrow” on the CW today is simply kidding themselves.

As Batman’s books became darker again in the 1970s, so did the Joker, who pivoted back to weird murders (just google the phrase “Laughing Fish” and see what come up) and dressing like a stereotypical ’70s pimp.

Which is to say, you can see why the Joker has stuck around. Writers can project all sorts of stories onto him. Even the Batman himself is not quite as flexible. The Joker can be silly, he can be a lone wolf, he can have henchmen, he can be brilliant, he can be goofy, he can be downright genocidal — it all works.

He was practically the last thing I just mentioned in Frank Miller’s “The Dark Knight Returns,” aka the 1986 four-issue miniseries where Batman comes out of retirement in a dystopian society. The series ended up one of the most influential (for good and mostly bad) superhero stories of the decade. The Joker has never been more sexually ambiguous than in that tale (which has not aged well), more dangerous or more murderous, taking out hundreds of victims before ending his own life in a manner than frames Batman for his murder.

Joker also was the subject of the still-controversial Alan Moore and Brian Bolland story “The Killing Joke,” a gorgeously rendered one-shot. It offers a possible origin story for the guy as a struggling comic with a family. “The Killing Joke” also covers Joker’s attempt to drive Commissioner Gordon nuts by shooting his daughter, Barbara (aka Batgirl), paralyzing her and possibly sexually assaulting her in the process — it’s left ambiguous in the story in an ill-guided attempt to be edgy.

The story is still considered in some corners to be a classic of grim-and-gritty ’80s comics; Moore since has wandered as far as possible from the thing. Bolland’s art is still masterful, but the story has aged poorly.

Audiences of the non-comic variety were treated to Nicholson as the Joker in 1989′s “Batman.” At the time, it seemed like brilliant casting (though, even back in ’89, there was a contingent of “This really should have been Tim Curry” types). Nicholson’s performance holds up in a “Yeah sure, this is one take on him” kind of way. But Prince’s soundtrack to the movie is far, far stranger than even grinning Nicholson could embody. This iteration of the Joker was an almost joyful artist of chaos.

Heath Ledger, on the other hand, de-emphasized the artist in favor of the chaos in 2008′s “The Dark Knight,” playing the Joker as a scarred, slightly hunched lunatic, quick with a knife and fond of blowing up hospitals. He was the kind of fellow who, as Batman’s butler Alfred puts it, “Just wants to watch the world burn.”

It is impossible to watch anyone but Ledger when he’s on-screen in that movie; no wonder he picked up a posthumous Oscar for it.

Between Nicholson’s and Ledger’s turns as the Joker, “Star Wars” actor Mark Hamill began a long stint voicing the clown in 1992′s “Batman: The Animated Series.” His performance defined the Joker for a generation of Saturday morning cartoon viewers. And then the less we say about Jared Leto’s tattooed, overly obvious turn as Joker in the genuinely terrible 2016 movie “Suicide Squad,” the better.

Now, we get Joker as sad-sack, incel-ish comedian, an origin based loosely on “The Killing Joke.” I have not seen the new film, but the previews seem to break cardinal rules of both the Joker and how our brains work in general. The more you know about a thing, the less scary it is. This is something comic book writers have always understood: The less of a sympathetic man there is under the clown face, the scarier and less human he is.

Besides sporting a design that makes people want to write stories about him, the Joker’s flexibility is part and parcel of his seemingly endless appeal as an ultimate baddie. Here is what fear, all kinds of fear, looks like.