Matthew Rhys had "genuine panic" that Bradley Cooper would die filming 'Burnt'.
The 40-year-old actor was worried about a scene where he had to save Bradley's troubled chef character from a suicide attempt, in which he tried to suffocate himself with a bag on his head, because his co-star is so "fearless" about pushing himself to the limit.
Matthew told Fox 5's 'Good Day New York': "There was no acting.
"I was required to do was tear the plastic bag from his face before he killed himself. So there was no acting required on my behalf, it was genuine panic. He's fearless in his improvisation."
Asked if he really was scared Bradley would suffocate, he added: "Yes. He had a very tight plastic bag over his head and obviously I don't recommend it."
Bradley, 40, previously admitted shooting the scene with Matthew meant they were "bonded forever" because it was so "beautiful".
He said: "It was late at night and we didn't have much time and the bag thing just sort of happened in one of the takes. It just feels vulnerable doing something like that in front of 12 people you don't know at all, but ultimately it was beautiful because Matthew Rhys who plays Reece was just incredible.
"We didn't really know each other at all and next thing you know, he's caressing me and calmed me down. We're bonded forever...
"We looked at each other after and we, why we both love doing what we do is to be able to really put yourself in imaginary circumstances and hope that accidents like putting an bag on your head and realising you can kill yourself happen."