Zac Efron revealed that filming for Extremely Wicked, Shockingly Evil and Vile was hard going and it was all down to his co-star.
The actor admitted he "almost had a mental breakdown" while filming the movie, but it may not be for the reason you'd expect.
The film depicts serial killer Ted Bundy's real-life story.
Speaking to ET, Zac Efron said co-star Lily Collins really impressed him - so much so he doubted himself.
"The first day of filming, I'll never forget," he remembered. "It's freezing cold in Kentucky, I smell like a thrift shop because my clothes were all from the '60s and I was just getting used to walking in this style.
"I walk in and I know my lines, I'm ready for the day and it was the first warm-up scene, and Lily was ready to go right then at rehearsal, perfect. Gave a perfect performance on the rehearsal, and I literally walked outside and I almost had a mental breakdown."
He added: "I was like, 'We didn't even get to rehearse and Lily is going for awards already?' [She's] so good! And I just sat at home and said, 'Zac, you gotta be better, you gotta be better, how can you do it?' And I figured out how."
The Ted Bundy film has already been making headlines before its release on Netflix on May 3 in the US and Sky Cinema in the UK, thanks to its depiction of the serial killer.
The team behind it are insistent that the film does not focus on the violence - one of the early concerns was it would glamorise the killings.
Efron speaking to the Guardian said: “I wanted to make this film for the victims.”
Efron also admitted he found it "almost impossible: to separate himself from the killer while filming.
Alongside Zac Efron, Lilly Collins stars as Bundy's girlfriend Liz Kendall and Kaya Scodelario as Bundy's wife Carole Ann Boon. John Malcovich stars as Judge Edward Coward, who sent Bundy down in 1979, and Metallica's James Hetfield also makes a cameo a police officer.
Extremely Wicked, Shockingly Evil and Vile is released on Netflix in the USA on May 3, and Sky Cinemas on May 3 in the UK.
