If you thought that watching the X-Men battle speeding space shuttles or the Men in Black taking on legions of aliens was the bravest thing you'd see on the big screen this summer, you'd be wrong. That honour belongs to Jade, the protagonist of Dutch writer/director Sacha Polak's English-language debut Dirty God. Played by newcomer and real-life burn survivor Vicky Knight, 20-something single mother Jade is attempting to readjust to life in the tower blocks of London after a vicious acid attack by her ex-partner, facing prejudice and discrimination at every turn.

'I remember seeing a young woman at a music festival who was severely burnt,' says Polak down the line from Paris, the latest stop on an international Q&A tour which has included screenings in association with Changing Faces, the UK's leading charity helping people with visible differences, as part of their #IAmNotAVillain campaign to promote sensitive and positive on-screen representations of scarring. 'I looked at her, and then I looked away. And I saw everybody around her doing the same thing. I realised that this woman deals with this every day of her life. She goes out to do grocery shopping, or whatever, and people stare at her. She can never hide from that. It's always there. She stayed with me.'

This experience left an indelible mark on Polak, sowing the seeds of a story she knew she had to tell. Even as she was attempting to make another film in the UK – one which she decided she didn't want to make – she and co-writer Susie Farrell were working on the screenplay for what would become Dirty God, interviewing young female burn survivors about their experiences. And the search for her lead actress began almost immediately. 'Before we had any money to make this film, when my producer and I were sleeping in youth hostels in Soho, I started the casting process. I wanted to know if there's someone like Jade out there.'

Indeed there was. Polak first came across Knight in a 2014 television documentary called – unknown to Knight while she was filming – Too Ugly For Love. 'That completely humiliated her, and she was very low,' says Polak. 'So she didn't want to talk to us at all. Lucy Pardee did the casting, and she stalked [Vicky] for a year trying to get in contact with her. Finally she called her with an unknown number and Vicky picked up, and she was able to explain that it wouldn't be a film about her but a fictionalised story. And she agreed to meet me. I was totally taken by her, I thought she was such a wonderful, beautiful person and such an inspiration.'

Indeed, much of the strength of Dirty God comes from the fact that while Jade's scars impact her life, she isn't defined by them. As we get to know her, we see that she is a vivacious, funny and sexy 20-something, as colourful, individual and flawed as the rest of us.

'That was why I showed the scars so close-up in the beginning of the film, so you could look at the person afterwards,' says Polak of the opening scene, in which the camera explores the lines of Jade's face in intimate detail. Another sequence shows Vicky indulging in webcam sex with strangers on the internet, a moment Polak felt was essential for her character. 'She's afraid of real contact with human beings. This is a way where she has some kind of control about it, and I felt it was liberating for her, in a way.'

Equally as liberating is a sex scene between Jade and her best friend's boyfriend Nez, played by newcomer Bluey Robinson, after their long-term flirtation spills over in a Moroccan hotel room. For Jade, who is in Marrakech after tracking down an affordable plastic surgeon on the internet, it represents genuine human connection. 'It was always important that Vicky felt safe around the other cast members,' says Polak, 'and it was very clear that they had a chemistry. Their lovemaking scene was difficult for them to do, because they are very inexperienced, and they really needed to trust me. But I think that scene is so touching and tender.'

Tender it may be, there's no doubt that sleeping with her friend's boyfriend is not the greatest decision Vicky could have made. But that fallibility was also an essential part of the character. 'There are a lot of films about women that are very one-dimensional,' Polak observes. 'Women are portrayed as these angelic creatures that are not very real. I like to show that there are different sides to every person, and when you have a female protagonist that she's not necessarily always likeable.'

That truth informs not only Jade's character but also her relationship with her mum Lisa (played by former Coronation Street star Katherine Kelly). As they attempt to adjust to life after the accident, neither woman is able to provide the other with love and support; just as Vicky acts out, ignoring her infant daughter Rae to go clubbing or engage with strangers online, Lisa finds it difficult to even look at her daughter, let alone provide her with comfort.

'I think Lisa is trying to be the best mum that she can, but they are all constantly afraid of each other's rejection,' says Polak. 'They behave in a way that's only making things worse. But for me it was important that in the end, even though the situation is not all of a sudden perfect, Jade steps up. And if you want to be a mother you have to take that role, grab it, and not lean back and let other people take over. That was important to me, that she could make that step.'

And as Dirty God goes out into the world, Polak hopes that audiences will connect with Vicky's journey to self-acceptance. 'The film is not mine any more,' she says. 'I feel very vulnerable that we are in between these big American films with enormous advertising budgets, and I have only one weekend to show the world that people want to see this film. But doing all of the advance screenings and Q&As has been very rewarding, in the way that people respond to Vicky's story. They cry and tell their own stories. That's why we make films; to show people that they are not so alone in the world, to touch them in some way. That is the job of cinema.'

Selected release from Fri 7 Jun.