Upon sitting down with Stephen Merchant (who plays a very scary SS officer indeed in the film "Jojo Rabbit," which opened Fantastic Fest on Sept. 19), one says to the towering actor the only appropriate thing: “You make a terrific Nazi.”

“Thank you,” Merchant says without missing a beat, in a manner that suggest he has heard this not-great joke before. “That is the nicest thing anyone has every said about me.”

(The following contains mild spoilers for “Jojo Rabbit.”)

New Zealand director Taika Waititi (who wrote and directed “Jojo Rabbit,” in which he plays an imaginary version of Hitler) is with us, too, wearing this impossibly cool-looking wrap-around shirt. It is the morning after the film screened, and anyone who saw Waititi at the Q&A would suspect he was having a very good time indeed. He did not confirm or deny.

But this film is fascinating, dicey territory for the comic savant. After acting in, writing and directing all sorts of shorts, TV work and a few features, Waititi broke out with the spectacular 2014 vampire mockumentary “What We Do in the Shadows,” in which he played a rather sensitive vampire with roommates.

His follow-up, 2016's “Hunt for the Wilderpeople,” was one of the best family movies of the decade. But he hit new heights of fame writing and directing “Thor: Ragnarok,” a genuinely effective action comedy and one of the most beloved movies in the Marvel Cinematic Universe. Dude is on a roll, which is how you get a movie like "Jojo Rabbit" made. (He is currently working on the next Thor movie, "Thor: Love and Thunder.")

In brief: “Jojo Rabbit” is the story of Jojo Betzler (Roman Griffin Davis, quite good), a kid growing up in Germany at the tail end of World War II. While his father is away due to the war, he is being raised but his mother, Rosie (Scarlett Johansson, as good or better than she has been in years), who is more than a little worried about her son's Hitler fanaticism.

Related: Fantastic Fest: ‘Jojo Rabbit’ tries its best in a world gone mad with hate

Jojo adores being a Nazi Youth and, well, a goofy Adolf Hitler (Waititi) is his imaginary friend. He spends much of this time hanging around his real life best pal, Yorki (played by a fantastic child named Archie Yates).

When imaginary Hitler encourages some rather severe recklessness in Jojo, our hero is benched at home, where he finds out that his mother is hiding a rather big secret in the family's walls, a secret played by a terrific New Zealand actress named Thomasin McKenzie. Rebel Wilson is Fräulein Rahm, an absurd Hitlerjugend instructor, while Sam Rockwell and Alfie Allen play the heads of the Hitler Youth camp who may or may not be lovers (Rockwell is typically wonderful but this whole plotline was not a great idea).

Waititi was inspired by a few things — the novel "Caging Skies" by Christine Leunens, his own experiences as a teenager thinking about the intersection between the Bosnian War and what happens when children find themselves in the middle of a hellacious, war-torn country, especially one run by actual fascists.

He is also pleased with the final product, which he sees as a timely cautionary tale about the power of hate that happens to have an absurdist Hilter at the center. “Not to toot my own horn,” Waititi says, “but the script was very tight.”

Casting was a much bigger deal, from the Nazis (Waititi on Rockwell: “He really is that charming in real life”) to the children.

“That was the biggest concern,” Waititi says. “I was too worried about the adults, there are lots of those. But casting kids is much harder.”

McKenzie came from, as Waititi put it, “New Zealand theater royalty,” but Davis didn’t come along until fairly late in the process.

At one point, I said “I cannot recall his name but—”

Waititi, smiling: “You are about to mention Archie Yates."

Me: “Yes!”

Waititi: “Yeah, he was wonderful and just as interesting as his character. He would try to call cut if something wasn’t working and at one point, during the big gun battle scene, he said ‘Does it have to be so loud?'”

That said, Waititi said it’s essentially filmmaking 101 not to know if the movie is working while you’re making it.

“No, you won’t know if it’s working until you are deep into editing, I think,” Waititi said. “When you are on set, you can't see the bigger picture. So if you finish the scene or come up with some brilliant shot or something, you might think, ‘Oh, I'm the world's greatest filmmaker.’ The saying goes a movie is never as good as watching the dailies and it's never as bad as the first assembly. Sometimes you look at the first cut of your film and think ‘Jesus Christ, I have really (messed) this up and there's a lot of work to do.'”

“Jojo,” to wit, went through 10 months of editing.

"I'd like ('Jojo') to create moments of reflection for people,” Waititi says. "And I don't mean debate, even though debate is fine. But I like films that just make you shut up and sit still and think about what you've seen and how that might apply to you or yours or to life. I hope people go home and hug their children after this and try and raise them better.”

Listen to Merchant and Waititi talk about the power of satire against figures like Hitler:

In a canny move, the Alamo Drafthouse's pre-show for the movie during Fantastic Fest was just clips of Hitler being made fun of through the ages, as if to remind audiences that a comedic portrayal of one of the 20th century’s most nakedly evil men wasn’t a new thing. Merchant gets animated when I mention this.

“People will say sometimes, ‘Is this kind of humor appropriate? Are you allowed?’" Merchant says. "And we know the great strength of comedy and satire, because dictators are terrified of being mocked. They hate it, because the thing they have is fear and respect. That's what they demand.”

Hence Waititi's conviction that the story is relevant in 2019.

“I like to think Hitler is still in pain,” Waititi says. “Every time we make fun of him, that somehow his soul is still floating around and it's still awful for him.”