The smoke and screams made him smile; the fiery cellblock looked identical to the one in the hand-drawn pages of Watchmen, the landmark 1985 graphic novel.
Snyder did not have to rely on memory - he had a rolled-up copy of the comic book on the set with him.
"Every day I think, 'I can't believe I get to make this come alive,' " Snyder said that afternoon outside Vancouver in an old paper mill that had been turned into a penitentiary for his $100-million film.
Watchmen finally reaches US theatres this week and will arrive as the most controversial superhero film ever made.
Snyder (43), an affable father of six, has been the picture of patience in the face of private setbacks and public challenges to the film, but while filming that riot scene, a wicked grin crossed his face.
"We're killing the comic-book movie; we're ending it," he said. "This movie is the last comic-book movie, for good or bad."
It was a playful pledge, of course.
The Dark Knight has hit $1 billion in worldwide box office, and no less than a dozen comic-book films are in the Hollywood pipeline.
But there is a undeniable end of times aura that surrounds Watchmen and goes beyond the doomsday plot and despairing Cold War vibe.
Watchmen has endured a long, ugly slog to the screen - legal battles over the property were resolved only in recent weeks.
The author of the graphic novel, Alan Moore, has ranted against Hollywood and its evils.
"I will be spitting venom all over it," he vowed last year.
More than that, comic-book fans, a notoriously strident constituency, have spent months debating whether Watchmen is the greatest film ever or the worst movie of all time.
"The unfilmable film" is how Snyder wearily refers to Watchmen, a superhero movie that will try to pull in a mass audience of non-believers with a 161-minute running time, no big-name stars, a hard R-rating and a hard-to-explain plot that takes place in an alternate version of Earth where Richard M. Nixon is in his fifth presidential term but the most powerful man on the planet is Dr Manhattan, a glowing superhero who spends much of his screen time showing off his iridescent blue penis.
For fanboys, this is the most eagerly anticipated and debated film opening since Tim Burton's first excursion into Gotham City in 1989, which signalled the beginning of the modern era of superhero cinema.
Unlike Burton's Batman, though, there is doubt about whether this grisly film will become a box-office hit.
Even the people involved in the film seem uncertain whether they have delivered a game-changing epic or a sordid spandex opera. "I just can't wait to see what people think of it, and I have no idea how it will be received," said Jeffrey Dean Morgan (known to Grey's Anatomy fans as dead Denny Duquette), who portrays the Comedian, a masked brute who murders for his government masters.
"I will tell you this: It's not a movie people have seen before. It's truly ambitious, and it's great to be part of that no matter what happens," Morgan said.
Snyder came into the film after the resounding success of 300, the 2007 hyper-reality sword tale (adapted from Frank Miller's Dark Horse comics) that blindsided Hollywood with its record box office for a March release.
That movie, like Robert Rodriguez's take on Miller's Sin City, was zealously faithful to the original comics, almost panel for panel at some points, and Watchmen has a similar allegiance.
That has led to industry jokes about whether Snyder is a "visionary director" (as he is labelled in the Warner Bros advertising campaign for Watchmen) or a comics fan with good eyesight.
Supporters of Snyder, though, write that off as talk by people who don't understand that Watchmen is a religious scroll.
Deborah Snyder, the director's wife and a producer of Watchmen, said the film has been "a million decisions made, and every one of them was to get the story on the screen with integrity".
Her team, she said, was not going down in history as the people who found the Holy Grail and dropped it.
"We feel a great responsibility."
This weekend, the Snyders are scheduled to be in San Francisco for a comic-book convention, one more stop to win the hearts and minds of fans.
"I don't think anyone who has never heard of 'Watchmen' can quite know what it means to these fans," Zack Snyder said. "There have been a lot of battles. More than people even know. But this is worth fighting for."
Snyder battled with studio chiefs to keep the film long, R-rated (bone-snapping fights, torrid superhero sex and the blue penis) and devoid of A-list movie stars.
On the latter, he said: "I wanted actors, not pop-culture names, and I got everybody I wanted."
The cast has Billy Crudup as the godlike Dr Manhattan, Patrick Wilson as the self-doubting Nite Owl, Malin Akerman as the wild-child Silk Spectre II and Matthew Goode as aloof genius Ozymandias.
Jackie Earle Haley plays the unrelenting Rorschach.
The film is a murder mystery that opens into a conspiracy tale, but like GoodFellas, it is about a tribe of outsiders who find themselves dragged down by betrayal in an increasingly cynical world.
The movie also has a Forrest Gump quality to it, with appearances by actors playing Fidel Castro, Mick Jagger, Leonid Brezhnev and Andy Warhol, not to mention an opening montage with Bob Dylan's The Times They Are A-Changin' as the soundtrack.
Snyder said it is advantageous that Watchmen did not get made sooner.
Only now, with the superhero cinema truly alive, is the genre ripe for snuffing.
"Twenty years ago, my parents wouldn't know who the X-Men were, and now everybody knows that stuff," Snyder said.
