Angelina Jolie is notoriously private.

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

The actress is starring in a new biopic, titled Maria, about opera singer Maria Callas.

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

"The character [Callas] has a lot of pain and they've of course seen me go through a lot of things, but they hadn't experienced me expressing a lot of the pain that usually a parent hides from a child," she said.

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

Jolie added that it was "a new way" of finding out how to be honest with her children about her feelings, "in an even greater way".

Written by Peaky Blinders creator Steven Knight, the film focuses on Callas's final years, in the 1970s, when she was living in Paris.

With Jolie taking on acting roles relatively infrequently in recent years, the film has provided something of a comeback narrative for her and could lead to an Oscar nomination for best actress.

Callas was a US-born Greek soprano, and one of opera's best-known singers.

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

The actress learnt to sing opera for the role, something she describes as "very physically demanding".

Training took around seven months, she said.

"We started with regular singing classes and it was challenging in many ways, but when the opera classes began, what it requires with your breathwork and your body and just the force of what you push through yourself, it's just a very different physicality."

Jolie, whose previous film credits include Changeling, Maleficent, Salt, and Mr & Mrs Smith, said she hasn't sung before, and was "actually quite shy about singing".

"It was probably one of the areas in my life that I was hesitant," she said.

But she indicated that it was also something she enjoyed.

"One of the greatest privileges of being an actor is you often are supported by a crew to try something and explore something you've never done and this certainly was most challenging," she said.

Jolie's sons Maddox, 23, and Pax, 21, have worked on a number of productions with her before, including her film Without Blood.

Both of them accompanied her at the New York City premiere of Maria in September, alongside their younger sister Zahara.

Jolie filed for divorce from Pitt in September 2016. The pair were engaged in a custody battle that resulted in Pitt being awarded joint custody in 2021.

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

On the set of the film, both Maddox and Pax were "very busy", director Pablo Larraín said. "They were good professionals," he added.

Jolie said that during filming, Pax recorded a lot of her singing practice "so he was with me in my early horrible days," she laughed.

"It's always good for your children to watch your mum not do something easily, but swear and fight and fail and have to try again," she said.

"So that's an important and beautiful thing."

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

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

"Jolie is absolutely spellbinding as Maria Callas, imbuing her with grace and resolve," said Sophia Ciminello of AwardsWatch. "She doesn't disappear into the role, she transcends."

Time's Stephanie Zacharek was less keen on her performance, however, saying Jolie "plays her subject as haughtily cool and deeply insecure, but captures none of her imperious charisma".

Hailed as La Divina, "The Divine One", Maria Callas began singing at 14 years old.

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

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

She spent her last years living largely in isolation, and died of a heart attack aged 53.

Larraín said he hoped his film honoured Callas's desire to popularise the art form.

"If this movie bring attention to opera from one to hundred to a million, it will be a success," he said.

"I don't know if there is an art form as strong as opera," Jolie added.

"The way it connects to the soul and the body, so of course it's for everybody."

Maria will be released in cinemas in the UK on 10 January.