Angelina Jolie is notoriously private.

But in a new interview with BBC News, the Hollywood star spoke candidly about her experiences working on set with her eldest sons, saying they saw “the pain” that she usually hides from them.

The actress is starring in a new biopicentitled Maria, about the opera singer Maria Callas.

Two of Jolie’s six children with ex-husband Brad Pitt, Maddox and Pax, took on roles as production assistants in the film.

“The character (Callas) is in a lot of pain and of course they had seen me go through a lot of things, but they hadn’t seen me express a lot of the pain that a parent usually hides from a child,” says she said.

“So they were there to witness some of it, but then we would hug or they would bring me cups of tea.”

Jolie added that it was “a new way” to figure out how to be honest with her children about her feelings in “an even bigger way.”

Written by Peaky Blinders creator Steven Knight, the film focuses on Callas’ final years in the 1970s while she lived in Paris.

Since Jolie has taken on relatively few acting roles in recent years, the film was something of a comeback narrative for her and could lead to an Oscar nomination for best actress.

Callas was an American-born Greek soprano and one of the most famous opera singers.

In Maria, a mix of Jolie’s own voice and original recordings by Callas are used in the singing scenes.

The actress learned opera singing for this role, which she describes as “very physically demanding.”

The training took about seven months, she said.

“We started with regular voice lessons and it was challenging in many ways, but when opera lessons started it was just a whole different physicality.”

Jolie, whose previous films include Changeling, Maleficent, Salt and Mr & Mrs Smith, said she had never sung before and was “actually quite shy about singing”.

“That was probably one of the areas in my life where I was hesitant,” she said.

But she hinted that she also enjoyed it.

“One of the greatest privileges of being an actress is that you often have a team supporting you to try and explore something you’ve never done before, and that was certainly the biggest challenge,” she said.

Jolie’s sons Maddox (23) and Pax (21) have already worked with her on several productions, including her film Without Blood.

Both accompanied her, along with her younger sister Zahara, at the New York premiere of “Maria” in September.

Jolie filed for divorce from Pitt in September 2016. The couple was embroiled in a custody battle that resulted in Pitt’s death She will be awarded joint custody in 2021.

The Hollywood stars also have daughters Shiloh and Vivienne and another son Knox.

On the film’s set, both Maddox and Pax were “very busy,” said director Pablo Larraín. “They were good professionals,” he added.

Jolie said that Pax recorded much of her singing practice during filming, “so he was with me in my early terrible days,” she laughed.

“It’s always good for your children to see your mother not doing something lightly, but cursing and fighting and failing and having to try again,” she said.

“So this is an important and beautiful thing.”

“Maria” is the third part of a trilogy of films about high-profile, complex women from Larraín, following his films about Jacqueline Kennedy and Princess Diana.

The film received mixed reviews, although critics generally praised its central performance.

“Jolie is absolutely mesmerizing as Maria Callas, bringing her grace and determination.” said Sophia Ciminello of AwardsWatch. “She doesn’t disappear into the role, she transcends.”

Time’s Stephanie Zacharek was less enthusiastic However, she said of her performance that Jolie “plays her subject as haughtily cool and deeply insecure, but captures none of her bossy charisma.”

Hailed as La Divina, “The Divine,” Maria Callas began singing at the age of 14.

One of her most famous performances was as Tosca at Covent Garden in 1964.

But the loss of her voice, possibly caused by dramatic weight loss, led to the premature end of her career.

She spent the last years of her life largely isolated and died of a heart attack at the age of 53.

Larraín said he hopes his film does justice to Callas’ desire to popularize the art form.

“If this film draws the attention of the opera from one to a hundred to a million, it will be a success,” he said.

“I don’t know if there is an art form as powerful as opera,” Jolie added.

“The way it touches the soul and the body is, of course, for everyone.”

Maria hits cinemas in the UK on January 10th.