"It sounds cheesy," he said, "but I wept a lot making this film because I was so moved by the actors" playing members of an arty family in Oslo who cannot talk to each other despite all their supposed sophistication.
"The actors are my friends. I know that they were being halfway a character and halfway themselves. And that they were also dealing with stuff," said the maker of "The Worst Person in the World", which landed the Norwegian two Oscar nominations and won newcomer Renate Reinsve the best actress award at Cannes in 2021.
Many critics that year said it also should have won the Palme d'Or top prize.
"We were a family too," said Trier, rehearsing his script around the kitchen table of the beautiful old wooden home in Oslo where the film was shot, itself a character in the film.
The heads that keep butting in Trier's on-screen family are the absent father, an arthouse filmmaker who has long been put out to grass, played by Swedish legend Stellan Skarsgard, and his stage actress daughter (Reinsve).
"I think a lot of families carry woundedness and grief," Trier said.
"And talk often doesn't help. It gets argumentative. We get stuck in our positions, the roles we give each other unconsciously."
Elle Fanning a 'mensch'
The bad old dynamics are changed by the arrival of Hollywood star -- Elle Fanning playing someone only millimetres from her real self -- a fan of the father, who comes bearing lots of Netflix dollars to revive one of his long-stalled scripts.
"We don't get too many Hollywood stars wanting to be in small Norwegian-language films," Trier joked.
But just like her character in the film, Fanning got the part through complete fandom, flying to Oslo between shooting the Bob Dylan biopic, "A Complete Unknown", and the new "Predator" in New Zealand.
"I am a massive fan" of Trier, she told AFP in Cannes, where the film is in the running for the Palme d'Or.
"I think 'The Worst Person in the World' is easily one of the best films in the last decade or even longer. It is just perfect."
"When Joachim sent me the script I read it and I was just crying and crying by the final page. It is so emotional," Fanning added.
"It's a very personal piece for Joachim and you can just feel that rawness in it."
Trier -- who comes from a family steeped in the Scandinavian film industry -- admitted it is all very "meta. You're making a film about a family with your filmmaking family. And you've got a meta Hollywood star."
But they are not that many parallels with his biological family.
"It's not like I'm throwing anyone under the bus. My whole family has actually seen the film and are very supportive," he said.
The filmmaker father, he insisted, is a mash-up of great auteurs such as Ingmar Bergman, Krzysztof Kieslowski and John Cassavetes.
Trier, 51, is famous for the bond he builds with his actors and he praised Fanning as the latest member of the family.
"She is a real mensch -- a really kind and collaborative, cool person," he said.
-Trier 'magic'
The "magic" that Fanning said Trier creates on set comes from taking your time, he told AFP, taking on the big themes with a light, humorous touch.
"Anyone who's had experience of therapy -- and I have -- will know that it's about the silences and letting things arrive. Very often is also the case with actors," said Trier.
"We had quite a few moments like that in the film actually. Renate would look at me and I look at her and I say, 'What was that? That was interesting.' And we don't talk about it anymore.
"But when people see it in editing, they go, 'Wow!'"
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