For her fourth feature film, director Lynne Ramsay gives the male action hero a serious makeover with a tender, blackly funny and vicious thriller about an ex-marine who is sent on a mission to save a young girl from a sex trafficking ring in New York. You Were Never Really Here stars Joaquin Phoenix in the lead role of Joe and is adapted from a novella by Jonathan Ames. Ramsay read the novella in a couple of hours and changed the final third by restructuring Joe's reaction to violence, 'I went through it very fast and I really loved this character. I liked the beginnings of this character – he's got this hunchback quality to him, he's almost a monster and you never know what he's going to do next. The quality of the writing and how unusual the character was for what would usually be a B novel or B movie stuck with me. One of the things I took from the book is that he was a very fallible man.'

Ramsay knew she wanted Phoenix in this role from the very beginning and soon after he agreed to do it, the pair began to collaborate on the psychological and physical development of his character. He moved around the corner from Ramsay roughly seven weeks before the shoot and any spare moment she had in between scouting locations in New York she spent working with him.

'That time was invaluable,' she says, 'It was cool that he did that because he prepared himself really early on. The way he looked physically and how big he was becoming before my eyes, and the way he walked was amazing. We talked a lot about the role, trying to get rid of any clichés. Joaquin and I talked a lot about masculinity and the tough guy, the knight in shining armour who rescues the girl. The male action hero normally saves the girl and we were doing the exact opposite of that. I wanted to redefine the clichés of those movies but for it also to be an intelligent film. It's about male impudence in a lot of ways, but in a way, he comes back to life. I thought of it as a bit of a Lazarus story.'

Ramsay's approach to violence and the tense action sequences set her film apart, 'I'd never done action sequences before, I'd never done anything like that. I tried to approach them from the psychology of the character. That's the kind of technique they used in silent movies – what you don't know and is left to your imagination is almost more potent.

'I thought [the violence] should be very mechanical at first which is why I brought in the surveillance camera. Then the violence becomes more personal to him, and then it becomes almost post-violence in the way that you don't see anything, but you can fill in the gaps. He's just moving forward. Right now, there's a lot of explicitly violent films and people know how it operates. What's more scary in this is that you don't see everything.'

Director Lynne Ramsay with actor Joaquin Phoenix

On set, Phoenix and Ramsay bonded over sharing music with one another and listening to Aphex Twin and Melkeveien's 'Peter Pan Death Wish' (the song featured in the trailer) to get them in the zone. Jonny Greenwood composed the electronic score for the film and Ramsay chose a soundtrack to match her character's mindset that skips and jumps to eerie effect. Ramsay also gave Phoenix audio clips of explosions and fireworks to listen to as an explanation of what was going on in his head.

'With the character of Joe I was trying to understand how memory works and how you recall things as flashes,' says Ramsay. 'Unlike flashbacks which tell a story I was looking at fragments and how people piece things together. The explosions represent how people remember.' An abusive upbringing and his time serving in the marines have left Joe broken and suffering from PTSD and Ramsay expertly puts the viewer in his disturbed mind.

Phoenix carried out his own independent research by speaking to men who work on sex trafficking cases. Ramsay watched documentaries on PTSD which explored the subject as far back as the Civil War, but she also focused on the modern world as part of her research. 'I've been watching a lot of documentaries about the state of the world today,' she says. 'I find documentaries really informative and I'm not on any kind of social media so it's my way of trying to understand the world. Nothing's black and white anymore, you can't trust anything anymore and Joe lives in that kind of world. He kind of looks into this abyss of darkness and it's just the tip of the iceberg. He lives in this world of uncertainty which I think is the world right now."

You Were Never Really Here is on general release from Fri 9 Mar. Lynne Ramsay chats to Edith Bowman in a live recording of her podcast Soundtracking, Glasgow Film Theatre, Sat 24 Feb.