The young actor shed 18 pounds (8.1 kilograms) for his role as drug addict Nic Sheff in the film and admits his skinny frame freaked out his family members.
"My mum was worried!" he tells W Magazine. "First, I'm in a movie where I was having sex with a peach ('Call Me By Your Name'), and then it was like, 'I got another movie!' She said, 'Great!' And then I had to tell her what it was about."
"Beautiful Boy" is based on Sheff's acclaimed book about his battles with drug abuse and his relationship with his father.
Chalamet reveals he researched drug use and abuse before auditioning for the part in the hope he would land the role.
"It was a script they'd been trying to get made for 10 years," he adds. "Every guy actor my age had gone up for it. I've been lucky, but a lot of the bigger Hollywood movies like 'Spider-Man', things like that, I didn't get. So, for 'Beautiful Boy', I did a lot of research and read about drugs, and I brought the books to my first meeting with the director."
"I could see in his eyes that he was thinking, 'This kid is nuts!' But I felt this movie - the subject of drug addiction - was so important. I wanted to make an anti-glorification-of-drugs movie. And I think we did."
Timothee met Sheff a week before he started shooting and the get together made him realise anyone can become an addict: "There was nothing about Nic that fit my stereotype of an addict," the actor says. "That was the learning grace of this movie - Nic is alive and well, but the reality is, it's a day at a time. You never really beat it."