Natja Brunckhorst, played Christiane F

The director Uli Edel and his team came to my school. I was sitting there eating an apple. Uli’s assistant came up and said: “We’re looking for girls for a film. Do you want to try out?” I said: “OK, since you’ve asked me, I’ll come.”

They put makeup on me and dressed me up like a junkie. There were eight girls and we stood together in a row. The camera went from one girl to the next. Bernd Eichinger, the producer, told me years later that, when it came to me, everyone watching the audition in the screening room gasped. I looked too young, I was very shy, and my voice was low – but, they said, there was just something about my presence.

During filming, they would put drops in my eyes to give me this drugged-out look. Uli would say: “Your eyes are half-closed and you are feeling a little sick.” Or: “Now, you’re cramping, and you’re feeling really sick.” I’d say: “OK.” Then I’d just do it. I wasn’t thinking at all. It was trusting and doing – that’s what acting should be.

We shot a scene on Kurfürstenstraße. Christiane, my character, is standing alone waiting for someone to pick her up. It was a long lens shot so the camera team was far away. A car drove up and I was about to get in when I saw, out of the corner of my eye, some of the crew running towards me shouting: “No! No! No!” And it came to me: this was a real guy, not an actor, trying to pick me up. I almost got into a car with someone who wanted to sleep with a 13-year-old.

After shooting finished, a friend’s mother told me she would never have allowed her daughter to do the film. I didn’t understand why. I grew up with my father and he knew I was very strong-willed. If I wanted to do the film, I would find a way. So my father said: “If you really want to do it, do it.” We never thought the consequences would be so huge.

Once the film was released [in 1981], I didn’t know what was going on. Everywhere I went, there was my face on the poster. I felt so exposed. Later, I left for London to go to boarding school, then I moved to Paris. Forty years on, I still meet people who say: “Oh, you were that girl? It’s so great you don’t take drugs any more.”

The film was based on We Children of Bahnhof Zoo, a book about the life of Christiane Felscherinow. A few years later, I met her. She was playing in a band at the Berlin punk club SO36. Someone introduced us and we said to each other: “Oh, so it’s you.” Then she went her way and I went mine.

Uli Edel, director

I had heard about the junkie scene in Berlin but I couldn’t believe it when I finally saw it: up to 100 kids hanging out at Kurfürstendamm station on weekends, ignored by the adults using the subway. Near the infamous Sound discotheque, underage girls who wanted to make a few bucks before they went back to the disco were offering quick sex to men cruising by in cars and trucks.

Neither Natja nor the other kids had acted before, and there had been no time for rehearsals. During our first week of filming they had problems hitting their light marks, following the camera dolly, and their performances were stiff. I hired a new cinematographer from the documentary world and told him: “Follow them with your camera, whatever happens. The kids should feel free in their movement and performances. Just try to get it like newsreel footage or for a documentary.” From then on, we had some focus problems and could only use simple lighting, but the kids felt liberated. Their acting was alive and convincing.

Kurfürstendamm station was in West Berlin but patrolled by East German train police due to crossovers there between east and west, and we weren’t allowed to shoot there. So our cinematographer sat in a wheelchair and hid the camera in a box on his lap, with an extended viewfinder sticking out of it. I pushed the wheelchair, following Natja cruising the station. We could only get away with doing this one time – in the final movie, some of the shots are still out of focus.

We also filmed in public toilets. One day, I was on a ladder filming Natja in a cubicle from above. I discovered a big bag of heroin in a wall recess. Bernd, my producer, had just been saying: “I don’t know how we’re going to finance the last days of shooting.” I said: “We could sell this. That would buy us a few more shooting days.” At that moment, a crazy-looking guy entered. He looked like a zombie on speed ready to attack. To stop him getting too close to Natja, I threw the bag at him. He caught it and left. And we kept filming.

• Amazon Prime’s remake, We Children From Bahnhof Zoo, is available now.