When the first Star Wars movie came out we were puzzled – people saw it six times a week. It captivated a generation

There was a time – believe it or not – when I used to say that Star Wars is just a movie, and people who are obsessed with it really should move on. Walk out of the theatre and forget about it – it's only a film.

Of course, a lot of people didn't do that. The first movie and its successors captivated a generation. The archetypal energy that made a lot of younger people want to actually live in the environment created onscreen was unique at the time.

When George Lucas and I began planning the first film, we had no idea what it would become; the kind of devotion it would attract. We planned to make this film that would capture the feel of the Flash Gordon serials of the 1930s, which we had seen on TV in the early-60s. When it started to steamroll into this huge phenomenon where people would go to the theatre to see it six times in one week, we were, at first, a little puzzled. The crossover from the sci-fi audience to mainstream filmgoers was truly surprising.

We soon realised that we had found a niche. Star Wars arrived in cinemas when a lot of science-fiction was gloomy and post-apocalyptic. There hadn't really been a hit movie designed to thrill the soul about the prospect of outer space as the final frontier since Forbidden Planet in 1955 – this was a few years before the first Star Trek movie, remember.

We knew there were hardcore sci-fi fans who would fill cinemas for a couple of weeks, but afterwards we thought it would settle down and maybe make its money back for Fox. Instead it just ran and ran, staying in some cinemas for nine months, which is almost unheard of today. If you tried to release Harry Potter in 50 cinemas and expand over eight weeks they would think you were crazy. Around 80% of box office is made in the first two weeks – then it's off to DVD.

So what was it that made Star Wars so different, so special? I can give you one small example of the kind of care we took when putting the film together.

While George was still working on the screenplay for the first film, we decided that sound was going to be an important part of Star Wars. So I went down to the USC cinema department, where George and I went to film school, and asked the head of sound if he had any finishing students who were particularly creative.

He gave me Ben Burtt, who became our sound designer. Ben worked for over a year collecting noises from all kinds of unusual sources, and we'd meet to listen and chat about them. By the time we finished shooting he had an enormous collection of sounds, which we used for everything from R2-D2's bleeps to the laser effect created when the space ships fire at each other.

We wanted Star Wars to be different, so we didn't want to use any stock "sci-fi" sounds. Ben gave us intricate combinations of different sounds that made the film stand out.

This was pretty much my role on Star Wars – smoothing the path for George to get on with his work: in other words, taking charge of actors and getting on with everything else that needs to be done during production. All films have micro-decisions to be made that can be irritating as a film-maker when you are trying to look at the bigger picture. George and I worked pretty well like this, and I think it was a good partnership that played its part in the movie's huge success.

Could Star Wars happen again today? It's hard to say because things were so different back then. The biggest difference is that it's now impossible to keep anything secret because there's too much information surrounding a film before anyone has even seen it. Even screenplays are published online prior to release.

In 1977 only a few thousand people knew about Star Wars when it first arrived in cinemas, but it spread like wildfire and soon there were queues everywhere. These days you would never see that because everybody gets into the first screening they want to get into, even if the movie is doing really well. Back then a film might open in just a few cinemas nationwide. When I lived in New York we used to drive around and see if there were any queues for the latest [Ingmar] Bergman. If there was nobody outside you wondered if it wasn't any good.

• Gary Kurtz is the producer of 1977's Star Wars and its sequel The Empire Strikes Back. He will be hosting a two-day Anatomy of Star Wars masterclass along with former 20th Century Fox vice-president Sandy Lieberson in London in May.