Cannes, France - Joaquin Phoenix stars in the darkly satirical movie Eddington, which premiered at the Cannes film festival on Friday, a biting take on America's culture wars set in a small New Mexico town.

The film by fast-rising American director Ari Aster is an unsettling but often amusing Western-style thriller set amid America's toxic politics and conspiracy theories.

Phoenix sparkles as a deeply flawed sheriff trying to keep order in the town of Eddington at a time of Covid mask mandates, Black Lives Matter protests, and inter-generational tensions.

"I think we're in a place right now where everybody is living in a different reality, in their own reality, and nobody can agree on what is real and what is actually happening," Aster told reporters in Cannes on Friday.

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"We've kind of lost one of the social forces that has always been at the centre of mass liberal democracies, which is an agreed-upon version of what is real."

Asked whether America was sick, the 38-year-old replied: "Yes, definitely."

"I think the final link to that old system (of agreeing on what is real) was cut during Covid," he explained. "I think that was the beginning of something big."

Best known for his previous horror movies Hereditary and Midsommar, Aster appears to be parodying everyone from gun-loving southern conservatives to virtue-signaling white anti-racism activists.

Emma Stone (La La Land and Poor Things) plays Phoenix's wife who gets sucked into a world of pedophile-obsessed right-wing conspiracy theorists.

Aster admitted to a sense of foreboding about America's direction and set out to dramatize it in his film, whose early social satire gradually gives way to much darker and more violent action.

Asked if America's polarized politics and the breakdown in trust in the media could be setting the country on a path to mass violence, he said: "That is certainly something I'm afraid of."

"It feels like nothing is being done to temper the furies right now," he told reporters.

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Aster's star-packed cast, which includes Pedro Pascal and Austin Butler (Elvis), is constantly filming each other, posting on social media, and messaging.

In one scene, Phoenix's character Joe Cross is asked by his deputy if he should share a video online of his incendiary speech.

"Don't make me think about it. Post it!" Cross snaps back.

The film is "about what happens when people who don't agree on what is real start coming into conflict with each other," Aster explained. "And that's what starts to possess everybody."

The film is competing for the prestigious Palme d'Or for best film in the main competition in Cannes, which will be announced on May 24.

Last year's winner, Anora by fellow US director Sean Baker, went on to win best picture at the Oscars.

Eddington is set to be released internationally in July.