Tilda Swinton plays more wildly adventurous characters than just about anyone in the movies.

Her mainstream career has seen her win an Oscar for Michael Clayton and feature in the Chronicles of Narnia movies, The Curious Case of Benjamin Button, We Need To Talk About Kevin and Trainwreck.

But Swinton has also worked repeatedly with such feted indie voices as Jim Jarmusch (in Broken Flowers, The Limits of Control and Only Lovers Left Alive), Wes Anderson (Moonrise Kingdom and The Grand Budapest Hotel) and the Coen brothers (Burn After Reading and Hail, Caesar!).

A fair achievement considering the stylish and apparently ageless 55-year-old with striking androgynous looks has never had any formal acting training.

"Don't call me an actress," Swinton says warmly. "It's not true. I'd feel an imposter if I was referred to as an actress."

What about "artist" then? That sounds like it fits someone whose career started with nine films with avant garde British filmmaker Derek Jarman, including Caravaggio, Aria and Wittgenstein, then a landmark gender-switching role in Sally Potter's art-house hit Orlando.

And Swinton has also featured in art installations – appearing to sleep in a glass case in London's Serpentine Gallery and New York's Museum of Modern Art – and she and filmmaker Mark Cousins have twice run a portable film festival that travelled through the Scottish Highlands on a truck.

But, no, Swinton rejects the term "artist" as well.

"I'm a performer," she says. "I feel more like a circus performer than an actress. Actress sounds a little pretentious."

Whether trapeze artist, clown or maybe even ringmistress, Swinton has come a long way.

Even geographically given she regularly collaborates with such acclaimed international directors as Italy's Luca Guadagnino (in The Protagonists, I Am Love, A Bigger Splash and just recently the horror remake Suspiria) and South Korea's Bong Joon-ho (Snowpiercer and the upcoming sci-film Okja).

But her latest movie is smack bang in the centre of mainstream Hollywood.

In the Marvel comic book movie Doctor Strange, Swinton plays a mystical healer known as the Ancient One who teaches an arrogant neurosurgeon (Benedict Cumberbatch) the power of magic after he loses the use of his hands in a car accident.

"The healing the Ancient One provides him with is not material healing," Swinton says. "It's spiritual healing. In fact, magic, the mystic arts, because the Ancient One is the Sorcerer Supreme.

"So it's all about magic and teachable superpowers. In many ways, it's not a superhero movie as such; it's more about sorcery."

Swinton's casting was controversial given the character was originally an Asian man in the Marvel comic books, leading to accusations of Hollywood whitewashing.

Swinton has a thoughtful response to the uproar.

"Anybody who's calling for a more accurate representation of a diverse world is talking about something incredibly important and I'm standing right beside them on that," she says.

"Until people see this film, I completely understand why they might misunderstand the reinterpretation of an older Asian man to be played by me as being problematic. But when they see the film, I think they'll understand that it was done for certain reasons."

Swinton says writer-director Scott Derrickson and Marvel Studios boss Kevin Feige explained they wanted the Ancient One to be an older Celtic woman because the character in the 1960s comics was a racist stereotype.

"The outcry about needing more diversity, particularly for Asian-Americans in an American cinema, is absolutely valid and important," Swinton says. "If that outcry has been made more visible by being attached to our film then frankly I'm all for it. All strength to the outcry."

Derrickson, best known for the Keanu Reeves remake The Day The Earth Stood Still and the Eric Bana horror film Deliver Us From Evil, says he wanted to avoid repeating the comic book's stereotype of the wise Asian mystic.

"My first decision was to make it a woman," he says. "I thought that was going to be an interesting way to avoid the stereotype and also the cliche of the wise old mentor teaching the young white hero. I initially thought I'd make it an Asian woman but every time I was working on the script, I kept feeling like I was writing a Dragon Lady.

"I know the history of American cinema and how poorly Asians have been represented and what the various stereotypes are and that is one of the most problematic ones. So I had to unshackle myself and go 'who could I write this for that could bring uniqueness and life?'

"I thought what's great about the Ancient One in the comics, especially in the later iterations? Well, it's the enigmatic, ethereal, other-worldly, quasi-duplicitous, mysterious, cryptic character. I thought, well, that's Tilda Swinton. That's the woman who could play this role."

Derrickson wrote a draft of the script as if he had cast her.

"I didn't ask her to do the movie," he says. "I just did a pass of the script for her and suddenly the role popped; it was great.

"So I gave the draft to Kevin Feige and said 'this is for Tilda Swinton. It has to be her. And if it's not her, I'm going to have to take another crack at rewriting this character because if she doesn't play this, I'm not sure anybody else could.' Luckily she read it and wanted to do it."

Once Swinton talked to Derrickson and read the script, she had not the slightest doubt about jumping into a comic book movie.

"I've been a huge Marvel fan for many, many years," she says. "I never imagined I would get a call to ask me about playing with them but I was super stoked when they got in touch with me."

While Swinton considers herself Scottish and has lived in the Highlands since the birth of her 19-year-old twins, she had a very English boarding school education.

Born Katherine Swinton, her mother Judith was Australian – Tilda comes from shortening her middle name Matilda – and father Major-General Sir John Swinton was a British Army officer. She went to school at the exclusive West Heath Girls' School in Kent – the future Princess Diana was also a student – before studying social and political science at Cambridge University. As well as performing on stage, she joined the Communist Party.

Her acting career started when she joined the Royal Shakespeare Company in 1984 before a breakthrough role in Orlando, Sally Potter's 1992 film about a nobleman who becomes a woman who lives for centuries. In her personal life, she is mother to twins with former husband John Byrne, a playwright and poet, and lives with artist Sandro Kopp.

So how is it you've been so successful without any acting training?

"With the exception of something like Marvel, that has something to do with the fact I've been a part of devising my work," she says. "It's very rare but always delightful when someone comes in and asks me to do something that I haven't actually been a part of creating.

"I tend to work as a producer and with filmmakers on cooking stuff up. It's me amusing myself really as no more sophisticated than any six-year-old dressing up as a dog or an old lady. It's really just me having a laugh."

But Swinton brings a political perspective to her roles as well.

"Art is a sort of sociological practice," she says. "If you're going to look with a fine tooth comb, I do come to my work with a kind of sociological perspective and certainly a political perspective. It's tricky for me to imagine a piece of work not being political in a certain sense but it is art."

In line with her politics, Swinton likes to work in collaborative groups.

"Something that really makes the world go round for me is working in a close-knit family," she says. 'It's the way I started making cinema when I started making films with Derek Jarman. I worked with him for seven years over nine projects.

"They were all made in a very tight-knit group. Then when he died in 1994, I was then as I still am, pretty pre-industrial.

"I've never learnt anything about acting. I'm really not interested in knowing anything about acting. But I'm really interested in filmmaking and working in a group.

"And fortunately after [Jarman] left the building, I was able to go on finding other filmmaking families to work with and I continue to work with them. Filmmakers like Jim Jarmusch or the Coens or Luca Guadagnino, they're my families."

Derrickson sounds like family too as he talks about Swinton.

"I've certainly never worked with an actress with less pretension than her," he says. "She's a person filled with deep, deep joy, a lot of wisdom and is somehow absolutely uncontaminated by Hollywood stardom.

"None of that matters to her. She cares about human beings, she cares about art, she cares about the things that make life worth living and she has a very reflectively beautiful view of things.

"If her acting career for whatever reason vanished, I don't think she'd be any less joyful. She loves doing what she does but she also does a lot of things. She's a special human being."

Doctor Strange is in cinemas now.