Actress Jodhi May cast as Selma James in Steve McQueen’s new BBC series, Small Axe

Jodhi May says Scarborough, in which her character has a romance with a 16-year-old boy, is supposed to be uncomfortable to watch

 Jodhi May and Jordan Bolger in Scarborough, directed by Barnaby Southcombe.

Actress Jodhi May, who stars in an uncompromising new film about a relationship between a teacher and a pupil, has defended it against critics who question its moral standpoint.

In British film Scarborough, out this weekend, May plays Liz, a teacher who is in a secret relationship with a 16-year-old schoolboy. She argues that her job as an actor was to “try to get into the head of someone in an indefensible position”.

Talking of the film’s contentious screenplay and several sex scenes, she told the Observer: “The minute you try to give a clear sense of the morality of a behaviour, you can’t inhabit the character. You know the audience is going to be intelligent enough to make up its own mind.”

Under the Sexual Offences Act 2003, it is illegal for an adult who is in a position of trust to have a sexual relationship with someone under 18 years of age. Prosecutions always make national headlines. In Merseyside this month, a married 43-year-old, Lydia Beattie-Milligan, who had worked in education for 25 years, was found guilty of arranging to meet a male pupil in a hotel room. Before she was jailed for two years, the jury at Liverpool crown court heard how she had sent text messages to the boy joking about him not going home that night. And six years ago 30-year-old teacher Jeremy Forrest was jailed for five-and-a-half years after embarking on a relationship with a 15-year-old pupil.

For May, who came to fame aged 12 in the award-winning 1988 film A World Apart, the role of Liz appealed precisely because of the challenges it posed.

“It is interesting to play a lead character who is really a victim of her unconscious,” said May, 44. “Liz is deluded and not aware what is really driving her. Thinking about it in that kind of depth helps if you are trying to get into the head of someone in an indefensible position.”

Jodhi May has just directed a short film for the portmanteau Everything I Ever Wanted to Tell My Daughter About Men. Photograph: Jamie Harvey

Scarborough, directed by Barnaby Southcombe, the son of Charlotte Rampling, follows the story of two teachers who take pupils on covert trips to the North Yorkshire seaside resort. The parallel relationships start on identical tracks, but the stories then diverge dramatically.

“It needs to be uncomfortable for the viewer,” said May, of the apparently sympathetic treatment of the four characters. “It is a very dark subject but it would be an error to simplify these characters. The balance of ambiguity is necessary. It is important to distinguish between the way a child would perceive a relationship like this and how it is seen by others.”

May is aware that audiences find the subject upsetting. In 2006 she appeared on a West End stage opposite Roger Allam in Blackbird, a highly praised play with similarly disturbing themes. “Every evening, Roger and I could hear the seats in the auditorium going up with a thud as people left, realising it was not the kind of show they had imagined,” she said.

Scarborough provides the perfect backdrop for relationships that “cannot exist in the real world”: She says: “The place became a sort of metaphor for a grand escape from the normality. There is a kind of crumbling, desolate grandeur, a promise of a dream that is broken and an underlying seediness.”

The film was inspired by a 2008 play of the same name by Fiona Evans, and by Southcombe’s memories of a schoolgirl contemporary who had an affair with a teacher. He has argued his film does not condone, but merely reflects reality. “In a culture of shaming, there is no place for teachers to safely voice their mixed emotions without fear of criticism and instant dismissal,” he told the Observer. “They are most likely wrong, they are certainly misguided and confused, but what they are mostly is human.”

May took the role, she said, because challenging leading roles for women seldom crop up. Yet she is soon to appear in several equally surprising parts. She plays the armour-clad warrior grandmother Calanthe in upcoming Netflix drama The Witcher, heralded as the new Game of Thrones, and will play Queen Victoria in the film Warrior Queen of Jhansi, about the 1857 Indian uprising against the British. May is also cast as Selma James, the American activist, in Oscar-winning director Steve McQueen’s much-anticipated new BBC series, Small Axe, about the British black activists of the early 1970s, who were known as the Mangrove Nine.

And in a busy year the actor has also just finished directing a short film which is part of a portmanteau feature created by 18 female directors and called Everything I Ever Wanted to Tell My Daughter About Men. “It’s an initiative to create more women directors,” explains May. “My film stars James Purefoy and Issy Knopfler, it’s set in the theatre and tells the story of a young actress who has an affair with an older actor and of its aftermath.”