Beau Is Afraidpitches Academy Award-winner Joaquin Phoenix into chapter after chapter of nightmarish scenarios. His terror-stricken protagonist runs an obstacle course through an urban hellscape,bachelor sex video flees a suburban prison, trips into an animated odyssey, and then into a sex scene sure to snap minds and score screams of laughter. But what was the most challenging sequence? As it turns out, Phoenix and the film's heralded writer/director Ari Aster have the same answer. 

In a paired interview with Mashable, Phoenix and Aster revealed the final sequence, in which Beau stands in a boat while going on trial for his crimes against his mother, was the most difficult for both of them. 

"At the end, that is set in a stadium," Aster said. "That was all made in CGI. So, we shot on a water tank, all green screen [around it.] So, the only thing that was there was Beau and a boat. Everything else was made later." 

Much of the visual effects in Beau Is Afraid, including its attic monster, distended testicles, and a frozen human figure, were created with prosthetics and practical effects. Relying on CGI to build the stadium was a new challenge for Aster. "That was a big, huge learning curve there," he admitted. "And that took a long, long, long, long time to get right. Longer than anything I've ever worked on. So that was the hardest thing." 

"That's funny." Phoenix responded. "I was gonna say the same thing but for a different reason."

The actor then painted the picture of what this experience was like for him. "I'm alone [in the scene], surrounded by green screen with a couple of guys in wetsuits rocking the boat, listening to an audio recording the actors had recorded during the shoot the previous night, and having to react to all of that." 

Phoenix had to physically keep his balance while staying in character, reacting to onlookers — including Beau's ferociously disappointed mother (Patti LuPone) and belligerent attorney (Richard Kind) — none of whom were present. Aster described it as "two days of shooting with [Phoenix] just screaming at nothing, as audio of Richard Kind boomed around the space." 

"That was very difficult," Phoenix concluded. 

Aster offered, "It was amazing to be on that set and see him doing it because he's in such a state of turmoil. To give that much when there's nothing to react to?" Aster then turned to Phoenix and said, "And I know you had to fight through whatever embarrassment there is there, because I know that was happening. That was pretty amazing…to just see him basically keep the faith that something would be there." 

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Keeping the faith was key to Aster and Phoenix's working relationship. When Phoenix was asked what it's like to see critics and audiences responding to Beau Is Afraid, his response spoke to his process as a performer.

"It's always strange for me, because I'm not thinking about an audience while I'm working," Phoenix said. "I mean, the only audience is Ari. And it feels like it's this really personal experience. And then one day, they'll go, 'Oh, by the way, we're showing this to a bunch of people.' It's a little bit like how Beau feels when he realizes that his therapy sessions are being recorded. It's kind of like — it seems like I thought that this was this private thing that we were doing, now you're telling me that anybody can watch this?"

"It works against me to consider an audience," Phoenix concluded. "It has to be just a personal experience."

Beau Is Afraid is now in theaters. Look for more interviews from the cast and crew on Mashable.