Iain Softley, director

I was working at Granada in the 1980s when I came across a photograph of Astrid Kirchherr and Stuart Sutcliffe while going through the station’s Beatles archive. They looked confident and interesting and I wanted to know more about them. I’d heard about the Beatles becoming a great live band while playing the clubs in Hamburg, but not the background story of Stuart, the group’s first bass player, and Astrid, a brilliant German photographer. Stuart died just on the eve of the Beatles becoming big, having left the group to pursue his art and be with Astrid. I was keen to get into feature films and had been kicking a few ideas around. This story, I decided, was the one I wanted to tell.

Stuart’s mother, Millie, lived in Sevenoaks, Kent – I think she was the fifth “M Sutcliffe” I found in the phone book. She and Stuart’s sister, Pauline, showed me some of his work and helped me make contact with Astrid, who was managing a wine bar in Hamburg. Over the years, I think a lot of people had tracked her down in search of Beatles gossip, but I told her that wasn’t what I was looking for.

Astrid invited Klaus Voormann along to our meeting – he was the boyfriend who had dragged her to see the Beatles in the first place, and he went on to be in the Plastic Ono Band and play bass on the Imagine album. I spent 10 days with them recording interviews which became the basis for the screenplay.

Ian Hart came on board early and would come in to read John Lennon’s lines opposite potential Stuarts and Astrids. I liked the idea of casting the two best-known actors in the film in those roles – Stephen Dorff and Sheryl Lee brought a movie-star quality to the characters the audience would be least familiar with. Ian had already played a slightly older Lennon in The Hours and Times, but that wasn’t the character I was after – I knew people who had met John in the early days and described him as angry, insecure and sometimes cruel. It wasn’t until I met Ian that I saw he could provide that energy. A lot of people say Ian looks just like John Lennon – he doesn’t really. It’s just that he embodies him so well.

For the soundtrack, we needed someone who could put together a band with star power. Producer Nik Powell was sitting on the toilet reading a music magazine when he saw the right name for the job. He ran out yelling: “We need to get Don Was!” Don put together this supergroup – Dave Grohl, Mike Mills from REM, Thurston Moore, Soul Asylum’s Dave Pirner, Henry Rollins and Greg Dulli of the Afghan Whigs. I said to Don: “Don’t let them listen to the Beatles’ versions. Just let them kick the crap out of the songs.”

Ian Wilson was a very experienced cinematographer. For the sequence where the band arrive in Hamburg and drive down streets near the Reeperbahn, we’d asked club owners to turn off all their 1970s and 1980s neon signs and leave on the 1960s ones. Instead, they did the opposite and demanded more money. Ian said: “Get the actors in the van and be ready to go in 15 minutes – trust me.” He got a camera assistant to go round the clubs saying: “Could you leave the 70s and 80s lights on and the 60s ones off?” Again, they did the opposite of what they were asked – and we got the shots we needed.

There were certain moments in the script that Astrid didn’t particularly like, where she thought people behaved in ways they wouldn’t have in real life. I said: “Wait until you see the film – I want you to be happy with it.” I sat beside her at the screening, a little apprehensive. She waited right to the end of the credits, when the screen went black, then turned to me with tears in her eyes and hugged me.

Stephen Dorff, played Stuart Sutcliffe

I thought: “How am I going to do the Liverpool accent?” I was auditioning against lots of English actors, but I’d just done a film where I played a British kid living in South Africa, and for that I had an incredible dialect coach called Julie Adams. She taught me a lot and I ended up working with her on this movie too and I still use her tricks now.

After I was offered the part, I spent a few weeks in Liverpool with Ian Hart, who had grown up there. I visited the pubs the Beatles used to drink in and John and Stu’s old art school. Ian was my right hand, my brother. He’s also a much better guitarist than me – I grew up playing piano, but I was perhaps better at playing bass than Stu might have been. I had to remember to occasionally hit a bum note or fall out of rhythm.

I met Astrid a couple of weeks prior to filming. She had quite an emotional reaction, which meant I did too. I was still a teenager and wanted to embody whoever I was about to play, and to be sure she believed in what I was doing. We stayed in contact afterwards – she was very supportive and always watched my movies.

I also became close to Pauline Sutcliffe, and even bought a couple of Stu’s original paintings from her. His work was reproduced for the film by artists – some of whom may even have studied under the same teachers as he did. I know recreating art in movies can come off a bit hokey, but I had people showing me what moves I needed to do and I just tried to get in the zone and convey his pain and passion in closeup.

Nine or 10 years after the movie came out, I met Clash frontman Joe Strummer in a club. He said: “You’re the geezer from Backbeat!” We ended up hanging out until the sun came up, drinking beer on the street out of brown paper bags. It’s an awesome memory. That’s what he kept calling me: “The geezer from Backbeat.”

• Backbeat is screening on 19 September at Picturehouse, Liverpool and 25 September at Picturehouse, Brighton. It is available on Blu-Ray