Philip Kaufman, director

I was still a teenager when I saw the 1956 Invasion of the Body Snatchers and was really affected by it. Two decades later, when I started to think about doing a new version, I went to see Don Siegel, who directed the original. While we were talking, his movie’s male lead, Kevin McCarthy, happened to pop in. They told me they’d wanted to make their version funnier, and with a really scary ending, but the studio had made them add the scene where the FBI solves everything. I cast them both: Don plays the taxi driver, and Kevin is the guy who runs into traffic trying to warn people, just as he did in the first movie. In my mind, he’d been running for 20 years from small town America to urban San Francisco. I wanted to show how paranoia had transferred to the city centres.

At the start, you see these gelatinous organisms leaving a planet that has been devastated. They’re going to do the same to our world – they come, they conquer and they leave. The organisms have some vague similarity to semen floating through space. I’d found this gel in an art store that I paid about $12 for, and we got the effect by dropping it into water and reversing the shot.

Cinematographer Michael Chapman and I walked the streets to get unstaged footage of people going about their lives – he had a camera hidden under a coat. The film provided a new context for the emptiness you can see in people’s faces as they’re waiting at a bus stop or looking out of a window.

We watched old film noirs and tried to recreate their look in colour. I’m not sure what category the movie falls into – the scene with Donald Sutherland and Brooke Adams having dinner and listening to jazz isn’t something you’d usually see in science fiction. I didn’t know Brooke could roll her eyes as she does when I cast her, but moments like that demonstrate the humanity these characters are trying to protect.

You see a banjo player on the street with his dog a few times – the music was played by Jerry Garcia of the Grateful Dead. Later we see the dog has the banjo player’s face – the result of Donald’s character striking the pod they were sleeping next to and causing an organic accident – man and beast have become one. For that effect, the dog was wearing a mask. We smeared something sweet on the front so its tongue came out through the mouth.

Ben Burtt, who had done the sound design for Star Wars, created the shriek made by the pods when they identify someone who’s still human, mixing pig squeals with other organic sounds. For the score, I chose an old college friend, Denny Zeitlin, who was a great jazz pianist but also a psychoanalyst. It’s the only score Denny ever did, but it’s a great one – he knew, as a shrink, how to scramble brains and keep us on edge. I used Amazing Grace to give a sense of hope towards the end which is almost immediately undercut. A screenwriter and director called Frank Pierson and I drank in a Scottish bar where that music was sometimes played by a guy with bagpipes, and each of us swore we’d be the first to use it in a movie – I beat him in that race.

Kevin McCarthy’s cameo was shot in the Tenderloin, the roughest area of the city. We did a couple of rehearsals surrounded by local people, and one guy was totally naked, lying with his head on the kerb, watching Kevin slamming into Donald’s car and shouting: “They’re coming!” The guy said: “Are you doing Invasion of the Body Snatchers? The first one was better.” We were in the middle of shooting and the first review of the film was already out.

When Kevin runs into the road, he interrupts a joke Donald is telling about the English Camel Corp, surrounded by Rommel in the Sahara. In case anyone’s still wondering, the end of that joke goes: “Men, there’s good news and there’s bad news. The bad news is that there’s nothing to eat but camel shit. The good news is there’s plenty of it.”

I knew and still know shrinks like Leonard Nimoy’s character. The scene where he’s signing books included many of my writer friends from San Francisco. The woman Jeff Goldblum argues with is my wife, Rose. Those people are part of the San Francisco I love, the world that is always endangered by pods – “pods” in this case being a metaphor for people who want to survive at your expense rather than coexist. I saw a cartoon of Trump in Donald Sutherland’s pose from the end of the film, pointing and screaming. That Maga world is the one I currently feel most endangered by.

Veronica Cartwright, played Nancy

Nancy’s the one who figures out what’s going on, and how to survive by staying awake and pretending you’re a grey person, walking around in a kind of fog. The scene where she makes the connection between the flowers appearing and people being replaced – “Why do we always expect metal ships?” – wasn’t quite working until I suggested having everyone talk over the top of each other, feeding into the situation. Philip always listens to people’s ideas – he’s a great actor’s director.

The newspaper article about webs shrouding the Bay Area that you see Donald Sutherland’s character Matthew cutting out in his lab is real. The week before we arrived for filming, spiders and webs had drifted out of the sky and veiled everything – it was just bizarre.

Donald was so funny. He’d decided his character had to have curly hair – I’ve no idea why. He had it set every morning and then again at lunchtime and during rehearsals he’d have his head covered in pink curling rods. Finally, towards the end of the shoot, he just said: “Oh, go ahead and perm it.”

Jeff, Brooke and I would go out to a restaurant after filming. One evening we jumped in a cab afterwards and Jeff was telling a story to the driver, who wasn’t responding at all. When we got out of the car all three of us looked at each other and said, “My God, he was a pod!” We found ourselves watching people just sort of drifting around, like they were pod people, not really experiencing anything. Suddenly it felt like there were pods everywhere!

When we filmed the ending, Phil walked Donald and I around the block and told us both something different about how the film was going to end. I don’t remember exactly what it was he said to me – probably that Matthew and Nancy were just going to wander off together. So when Donald pointed at me and did that scream, I flipped out for real. He did make some sort of a noise, though obviously other things were later mixed into it, but it was totally terrifying, and what you see on screen is the reaction I had to his sort of betrayal.

• Invasion of the Body Snatchers is out now on 4K UHD Blu-ray from Arrow Video