'I grew up in Yorkshire, in a very isolated place,' says actor-turned-filmmaker Francis Lee about the deeply personal inspiration behind his blistering rural drama debut God's Own Country. 'I escaped when I was 20, moved to London and went to drama school. But there was something about that landscape, those people and that way of life that cast a real shadow. When I decided to give up acting and make films, it felt a very natural place in which to explore stories.

'I'd never seen that part of the world, something I was so familiar with, depicted on screen in the way in which I saw it,' Lee continues. 'I always felt like I was looking at the countryside as a tourist vista or a pretty postcard; it was pastoral, bucolic, calming, relaxing, slow. And I'd never seen it that way. For me, it had been cold, harsh and isolating.'

And it's against the backdrop of this brutal landscape that young protagonist Johnny (Josh O'Connor) works his family's struggling farm; a task made even more difficult following the recent stroke of his father (Ian Hart). Geographically isolated and emotionally repressed, he numbs his physical and psychological pain by getting blind drunk and engaging in aggressive sex with anonymous lads. The arrival of sensitive Romanian farmhand Gheorghe (Alec Secareanu) changes everything, igniting a spark in Johnny that's not easily ignored.

Both O'Connor and Secareanu turn in astonishing performances in two challenging roles; a result, says their director, of careful preparation that went far beyond having them work long shifts on local farms.

'My experience as an actor taught me what an incredibly difficult job it is to stand in front of a camera and make yourself vulnerable,' Lee explains. 'I knew that I had to build incredible trust with the actors, and give them what they needed to push themselves. With Josh and Alec, we built their characters from scratch, from the moment they were born until the moment we meet them in the film. We plotted everything in great detail – significant events, how they were day to day, what their school life was like, what their family was like, their sexual encounters. So by the time we started to work together physically, the boys were totally immersed in who [their characters] were.'

This carefully structured approach extended into filming. 'I shot the film chronologically, because I felt it would really help the boys' relationship,' says Lee. 'Each scene felt like building blocks that impacted on the next. I kept the boys separate until they met on screen for the first time; I knew that would add an extra layer, an extra frisson. And then once they had met and their relationship was developing, I moved them into living in the same house and they became very good friends. For me, it's all in the detail and authenticity of character and place.'

Indeed, authenticity is the beating heart of God's Own Country, from its richly drawn characters and multi-layered natural soundscape to its production design – all costumes and props were sourced from local farms and shops – and portrayal of the Yorkshire countryside as far removed from the familiar rolling hills and gentle fields.

'That was absolutely my intention,' says Lee. 'Very quickly, cinematographer Joshua James Richards and I realised that we didn't want to tell this film in wide shot. I knew I wanted this story to be incredibly immersive and emotional, and that I wanted to discover Johnny's world through his eyes. So the camera is very close to him all the time. We didn't want to see the landscape, rather the effect it has on the people within it. So we see him traipsing through the mud, how cold he is, how his hood is up and his head is down. He is battling that landscape, rather than enjoying it.'

Some enjoyment does finally come, however, as Johnny and Gheorghe embark on an increasingly intimate relationship which includes some sex scenes that, while being entirely true to the story and its characters, are undeniably explicit. Lee, however, is unconcerned that these moments will become a bigger talking point than the film itself.

'To me, the sex feels very much part of the world,' he says. 'I always knew that Johnny was going to go on what is, for him, a transformative emotional journey,' he says. 'But I knew he wouldn't sit around, navel-gazing and talking about his feelings. So I showed that transformation in the way he reacts to sex, that change that happens within him through intimacy and touch. I had to be truthful with the way I depicted that, visually. Also, I feel that they are no more explicit than what you see on TV; I think the only difference is that they are two men.'

The film arguably also pushes boundaries in the way it realistically depicts the full range of male emotion which, in cinema, traditionally falls into the category of either square-jawed stoicism or campy melodrama. Lee, however, challenges the notion that his film is gender specific. 'I know I decided to write a film about two men, but I've never distinguished emotion by gender,' he says. 'The starting point for me in writing this story was working out the toughest thing that I have ever had to go through is falling in love, and making myself vulnerable enough to love and be loved. That was what I wanted to explore.'

As an exploration of love and a study of a little-seen part of British life, God's Own Country is both a sensitive and accomplished piece of filmmaking and a strikingly bold debut. Its filmmaker, however, laughs off any suggestion that his choice of first project could be described as brave. 'I could only make this film the way in which I made it,' Lee says, 'and I was incredibly supported by the BFI and Creative England who only challenged me to go deeper.

'Everybody who makes a film goes through the same emotional journey,' he continues. 'It takes over your life. I'm just really thrilled that people are finding things in the film they really like, and that resonates with them. That, to me, is the biggest compliment you can get.'

God's Own Country opens the Edinburgh International Film Festival on Wed 21 Jun. General release from Fri 1 Sep.