By Helen Bushby Entertainment reporter in Los Angeles

Image copyright Getty Images

Stringed instruments have long been used to make people feel edgy.

Think of a double bass rasping "duh-nuh, duh-nuh" as Jaws approaches, or the screeching violins of Psycho as terror strikes.

There's something visceral about their sound, which can make your hair stand on end during a horrifying film sequence.

In Joker, the cello conveys the inner turmoil of Joaquin Phoenix's character Arthur Fleck, as he contorts into a murderous clown.

The film's soundtrack is the work of Icelandic composer and cellist Hildur Gudnadottir - the only woman nominated for best score at this year's Oscars.

Anyone who's seen the film will know the music plays an integral part.

Image copyright Warner Bros

"Joker is the story of basically one person, so it felt fitting that one instrument was leading the way into his head, leading us forward," she tells BBC News.

Of all the instruments at her disposal, she chose the cello. "It's my instrument, so it was convenient as I played it on the soundtrack," she says.

Hildur was lured into working on the Joker origin film by director Todd Phillips, and she recalls: "The script was just fantastic - it struck me very hard."

She collaborated on Arthur's transformation into Joker with Phillips and Phoenix, the favourite to win best actor at the Oscars.

"I fell in love with the film and wrote all the main themes before they started shooting, so they were able to use that music, and I was able to be a part of that."

Hildur's music was pivotal to a key early scene in the film - the "bathroom dance" where Arthur, dressed as Joker, begins to inhabit his alter ego.

Her score triggered an unexpected response in Phoenix.

In the half-light of a bathroom, he slowly twists his arms in a series of oddly balletic poses as a haunting cello solo plays. It makes for compelling, if creepy viewing.

"Joaquin was basically responding in real time to the music - the scene wasn't scripted like this at all," she said.

"The scene in the bathroom where he's being led by the music and improvises a dance, it was just basically his reaction to hearing what I'd written.

"A lot of his performances that happened after that point were because that moment just worked so well, and the music really kind of supported him."

Indiewire reported that Hildur's score became "the actor's muse" and "thereafter, he could be seen listening to the music in his ear piece on set throughout the shoot".

Hildur, whose work includes composing the score for last year's TV series Chernobyl, has already caused a stir with her recent winning streak at the award ceremonies.

So far she's notched up a Bafta and a Golden Globe for Joker and an Emmy for Chernobyl.

But it won't have escaped many people's attention that the lack of diversity during awards season is still a huge issue, with the Oscars, Baftas and Golden Globes coming in for much criticism.

In the 84 years that the Oscars have awarded best original score, only seven women including Hildur have been nominated, and only three have gone on to win.

The most recent female winner was British composer Anne Dudley, for The Full Monty, way back in 1997 and the academy has yet to nominate a woman of colour.

Last year's Celluloid Ceiling Report, by San Diego State University's Centre for the Study of Women in TV and Film, stated:

• Just 7% of composers working on the top 500 grossing films of 2019 were women
• On films with at least one female director, 16% of composers were women
• On films with exclusively male directors, women accounted for 6% of composers

Equality is evidently still a good way off for female composers, but has anything changed in the short space of time since Hildur was nominated for Joker?

"Obviously, we could definitely do with having more women, for sure, but what I do find fantastic is that after the run I've had in the last few months, there's been a lot of discussion happening as result of that," she says.

"I'm the first woman to have been nominated and win like this for a long time, so people have been talking about it a lot.

"That's been really wonderful, because I've just been hearing from music supervisors and editors that directors are, as a consequence, asking for more female composers to work on their films.

"That hasn't really happened before."

One of the things Hildur enjoys about her music is that it allows her to explore many aspects of her personality.

"I enjoy doing the darker side of film. My personality is rather cheerful, I like to laugh, but every person has a dark side and a light side, and luckily for my family, my lighter side is the main side and the darker side is in my music."

With a female composer, Joker is helping carve new ground for films inspired by comic books and super-heroes.

Captain Marvel, starring Brie Larson, also has a female composer. Pinar Toprak, from Turkey, who wrote additional music for Danny Elfman on 2017's Justice League, is the first woman to score a Marvel film.