Billy Crystal returns to his 30s in 'Before.' Even his wife thought he looked 'unbelievable.'

If you did a double take seeing a younger-looking Billy Crystal in Friday's episode of Before, you're not alone. So did his wife!

Crystal, 76, broke down the wild process to Yahoo Entertainment of how he de-aged for a flashback scene in the psychological thriller. The actor said he's not a fan of double casting, meaning they would have cast a younger actor to play his character Eli, so he relied on "amazing" deepfake technology from Deep Voodoo to pull it off.

"I think of what I went through with [filming] Mr. Saturday Night back in 1991 — my head weighed about nine pounds," he joked, referring to the heavy prosthetics he wore to appear older.

Deep Voodoo is an AI entertainment startup founded by South Park's Trey Parker and Matt Stone. Weeks before filming the flashback scene for Before, a crew from Deep Voodoo took a video sample of Crystal's face. Crystal told them he wanted to look like he did in 1986's Running Scared.

"I was in my 30s then and that's the age we wanted [Eli] to be," Crystal explained. "This system creates a digital mask [you] put on your face. ... You had to wear this heavy black kind of wig that's a sample. And then the computer takes that and just does everything to finish it up."

When they yelled cut on set, Crystal got to see the younger version of himself in real time.

"I ran over with a stupid thing on my head and there I am from Running Scared. It was crazy. And my wife Janice was on the set that day ... and she's watching me watch it and I said, 'This is amazing.' And she says, 'You look unbelievable. I'm in room 445. That was spectacular,'" he recalled.

Crystal said he's "grateful" technology like Deep Voodoo exists although he was worried at the initial suggestion.

"We were nervous when [writer and creator] Sarah Thorp sent me the idea. I said, 'We can't double cast. This is gonna be hard to pull off,'" he recalled. "I don't want someone young playing me. It never connects with the audience. You can do it on stage in a theater ... but not on film. This technology came into being and I'm really grateful that it worked as well as it does."

Before follows a child psychiatrist named Eli (Crystal) who starts working with a troubled young boy named Noah. The child seems to have a haunting connection to his past. It's a project unlike anything Crystal has ever done before.

"It's one of my favorite roles I've ever had to play," the actor said of his dramatic turn. "Not just because it's different, but because it's a really great arc and a really interesting character. A chance, at this point in my career, to stretch and grow more was thrilling to me."

Before is streaming on Apple TV+ now.