Angelina Jolie's Vocal Coach Reveals Why She Was 'Perfect' to Star in “Maria” Despite Never Singing Before (Exclusive)

Eric Vetro tells PEOPLE about working with Jolie for around seven months in preparation for Pablo Larraín's new biopic about opera legend Maria Callas

Vocal coach Eric Vetro was quite impressed by Angelina Jolie's portrayal of Maria Callas.

Jolie stars as the late opera legend in Pablo Larraín's new biopic Maria about Callas' final years living in Paris before her death at age 53 in 1977, and Vetro worked with the Oscar winner on her vocal performance in the film for around seven months.

"She started from ground zero. She had never sung before," Vetro, 68, the go-to coach for stars from Ariana Grande to Timothée Chalamet, tells PEOPLE of Jolie, 49, whose voice is blended with Callas' original recordings throughout Maria. "I don't think I've ever seen someone make such a huge difference from the first day to the last day."

Related: Moonlight Director Compares Angelina Jolie's Maria Performance to Denzel Washington as Malcolm X: 'So Striking'

Vetro recalls Jolie feeling "a little nervous and shy about singing in the beginning," but he quickly learned just how determined she was to nail the part. "Her desire to do a great job overrode anything that might stop her."

The coach is used to working with novice vocalists, but training someone to sing opera is a unique experience, as there's rarely a steady beat to follow. "It ebbs and it flows. It goes a little faster here, a little slower there," he says. "So to do that, I mean, you have to really be musical."

At one point during their sessions together, Vetro heard Jolie sing "O mio babbino caro," a soprano aria from the 1918 opera Gianni Schicchi — and he was blown away.

"It was the most thrilling day of all of our time spent together," he recalls. "I was like, 'This is it. She can do it. She's the perfect person to play this role.' And I'm someone who loved Maria Callas — I really did. She was one of my favorite opera singers, and so it really touched me that she was able to do it like this."

While Vetro was impressed, he wasn't exactly surprised. Throughout Jolie's career, she's proven herself quite versatile by acting on screen across many genres, directing several films of her own, producing Broadway's Tony-winning The Outsiders, working as a humanitarian and raising six children.

"I feel like anything she decided to do, she probably could do. She's a remarkable human being," says Vetro. "She really inspired me to always give 150% every minute of the day, because she does."

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories. 

Maria is in theaters now and will be available to stream Dec. 11 on Netflix.

Vetro recently offered up his skills for BBC Maestro's "Sing Like the Stars," an eight-hour series of videos featuring advice from his star students Sabrina Carpenter, John Legend and Marissa Jaret Winokur. The course is available now.