The "Call Me By Your Name" star plays a drug addict in the movie and to prepare for the role, he lost 20 pounds (nine kilograms).
Timothee was being closely monitored before and during production, but Groeningen worried the 22-year-old would "go into a dangerous place" if he got too skinny.
"It helped him physically feel it and hit some sort of rock bottom (for the role)," Groeningen says. "I mean, he was very closely followed but at some point, we had a feeling he was pushing it further than we wanted, because he was so committed to it. We did have to say at some point, 'Timmy, stop. It's great. We don't want you to go into a dangerous place'."
Chalamet appears alongside Steve Carell, who plays his father in the film, and "The 40 Year-Old Virgin" star reveals their acting chemistry was instant, so it was not difficult to show genuine affection towards each other.
"We just naturally became friends," Carell explains. "There weren't any exercises that we needed to go through in order to generate that affection. I think we had it almost instantly."
"Yeah, I feel the same way," Chalamet adds. "I felt very... just taken in. I felt a strong warmth from Steve and from Felix, our director, and then the scenes with (co-stars) Amy Ryan and Maura Tierney as well. I felt a very paternal quality coming from them."