It's a risky move, playing an adult who sleeps with a 15-year-old - you have to get the dynamic just right. But it's also a challenge, which is why Alexander Skarsgard was "intrigued" enough to sign up for Diary Of A Teenage Girl.

The actor plays 35-year-old Monroe, who becomes involved with his girlfriend's daughter, Minnie. Sure, he's predatory and sleazy - but not straightforwardly so. He's also secretly vulnerable and childish, and sometimes, thanks to Skarsgard's portrayal, almost a sympathetic character.

"I was really intrigued by the character of Monroe. I thought, 'How can you make this guy real? How can you make this relationship interesting for the duration of the movie, so it's not too predatory?'

"Even though he's obviously ultimately responsible for what happens, it was important to make that journey interesting," says Skarsgard, 38.

As Minnie (Bel Powley) becomes aware of her sexuality, her mother's boyfriend starts to make advances. The two begin a covert relationship, with Monroe driving it forward at the same time as he seems overwhelmed by Minnie's personality and the strength of her desire.

"At moments, I wanted it to feel like he was almost confused, and almost like a young boy," the actor explains. "He's a teenager in certain ways."

Skarsgard is the oldest son of Swedish actor Stellan Skarsgard, who's appeared in the likes of Good Will Hunting and Avengers Assemble, but it wasn't a straightforward decision to follow his father into acting. After starring in a Swedish movie at the age of 13, he became famous in his home country - but didn't like it, and retreated from showbiz until his mid 20s.

However, his love of acting eventually drew him back in, and he landed a role in 2001 Ben Stiller comedy Zoolander. Since then, it has been diversity and interest which has driven Skarsgard's career, rather than a desire for fame. He's been a vampire in True Blood; Lady Gaga's boyfriend in the Paparazzi music video; even the English voice of Moomintroll in a Moomin movie.

And now he enthuses about Monroe: "I felt like I hadn't played a character like that before, and I didn't quite know how to approach it... That's what got me really excited about it."

The film is a curiously innocent exploration of female sexuality, despite the drug use and nudity and sex, which meant it was given an 18 certificate. The director, Marielle Heller, has objected to its high rating, accusing the British Board of Film Classification of gender bias, and arguing that "they have missed the point of the film and its message".

Skarsgard, who has spoken out about the industry's hypocritical approach to women and sexuality, is fully on board with Heller's vision, saying: "I've never seen this portrayal of a young girl, this coming-of-age story that was so brave and unapologetic, in a way."

On top of the challenge of playing Monroe, after reading the script, it was the film's importance which attracted Skarsgard. "I was blown away by it," he explains. "I thought it was so smart, so funny, so beautiful, and important without being preachy or didactic."

Powley was also drawn in by the script and how it handled female sexuality; as part of her audition tape, she sent a video message to Heller explaining how she related to Minnie.

"I've never read such an honest portrayal of what it is like to be a teenage girl, especially when it comes to sexuality," the London-born actress explains. "And then I suddenly thought, actually people don't even - in life - talk about female sexuality. For girls aged 13 to 16, it's such a taboo subject, and I really related to that feeling of thinking about sex and going, 'Oh, does that make me a freak?'

"I think with boys, it's so easy to talk about it. There are so many movies about boys getting laid, boys losing their virginity, and it's very normalised. But for women, it's quite rare to have that in a movie. So I wanted to be part of that conversation."

Though Bel is 23 to Minnie's 15, acting out sex scenes intended to portray underage sex could have been awkward. But, says Bel, this wasn't the case: the scenes felt "natural", "not gratuitous".

"We talked with Mari [Heller], and went through Minnie and Monroe's relationship, so we could plot it out emotionally. So each sex scene we were doing, we knew exactly where our characters were in their heads, and it made sense. You know, if it makes sense, it's not uncomfortable."

Next year, Skarsgard will be seen on the big screen as Tarzan - the "polar opposite" of sleazy Monroe, to say the least.

Shooting the film was also an entirely different experience, he recalls. Diary Of A Teenage Girl was more hands-on, a "super small, intimate crew of 20 - you're lugging stuff around together and you're shooting out on location and running around", he explains.

But then it was off to Watford, near London, to shoot Tarzan, "and it was the biggest set I've ever seen in my life".

"It was so great, I loved it," he adds.

For Skarsgard, it's all in the variety. Each role brings a different experience, a new challenge: "That's what's so amazing about what we do for a living."

:: The Dairy Of A Teenage Girl is released in cinemas on Friday, August 7