What scares John Carpenter? Over his four-decade career, the self-described “master of horror” has directed so many classic big-screen thrillers – Halloween, The Thing, They Live – that you might imagine he's immune to fear. But when recently faced with the prospect of performing music in front of audiences for the first time, the 68-year-old admits: “I was terrified!”

Carpenter's fear dissolved on the opening night of his current tour, in which he and his band play selections from his film scores alongside new songs from two recent albums. "I started playing and I realised, ‘This is easy!’” he recalls. "As a director, you stand behind the camera and you sweat and cry and scream for months. Performing, you play for an hour and half, and you're done! It's so fun."

Although Carpenter describes his live show as a “career retrospective”, its impetus came from a recent surprise. A few years ago, he started jamming habitually in his home studio with his 32-year-old son Cody, just for fun. They improvised in the vein of Carpenter's pioneering movie soundtracks, which he composed and played on synthesizer. The ominous tones and tense chords influenced film composers as well as underground musicians, who built on his innovations to create evocative instrumentals.

Not long after the father-son jam sessions began, a lawyer Carpenter hired to represent his soundtrack work asked if he had any new material to share. "So I gave her the improvised music I had done with my son, and lo and behold, we got a record deal," he says. Adding his godson Daniel Davies (the son of Kinks guitarist Dave Davies) into the mix, Carpenter has made two albums for indie label Sacred Bones, Lost Themes and Lost Themes II. The latter gave rise to his first tour, with the trio accompanied by musicians who previously backed Jack Black’s Tenacious D.

Both Lost Themes volumes show that Carpenter's powers as synth player and mood creator are undiminished. Using simple melodic lines and dramatic, repetitive notes, he crafts wordless narratives that, true to their name, sound like scores in search of images. “They're soundtracks for movies that exist in your mind,” he says. “Everybody has something playing in there. My recommendation to you is to get in your car late at night, drive the dark city streets and put on Lost Themes. You'll experience it."

Music has filled Carpenter’s mind for even longer than he has made movies. His father, Howard R Carpenter, taught classical music in Kentucky and played violin for the Nashville Strings, supporting Roy Orbison, Johnny Cash and Brenda Lee in studio sessions, some of which the young Carpenter witnessed as a child. He eventually took his own stab at violin, but was never destined for that, he says: “It’s a really hard instrument to learn. To sound good on violin is nearly impossible.”

Instead, Carpenter turned to keyboard and guitar, joining a high school rock group that played frat parties on weekends for extra cash. But his first love was film, which hooked him at eight years old, when he marveled at the 1956 science fiction movie Forbidden Planet. “It was stunning to me. I had never seen anything like it,” he remembers. “It had an all-electronic score, no orchestration. Everything about the movie was transformative to me. It made me want to become a director.”

When the time came to choose between music and movies, Carpenter followed his childhood dream, moving to Los Angeles to study film at the University of Southern California. Soon after graduating, he made a string of movies – 1974's Dark Star, Assault on Precinct 13 in 1976 and one of the most successful independent films ever, Halloween, in 1978 – for which he created scores by improvising on his synthesizer while watching final cuts.

Carpenter made his own soundtracks out of financial necessity. As he puts it: “I could only afford me.” But his admittedly limited musical vocabulary was a good match for his storytelling style. “Simple, repetitive themes are very effective,” he says, citing the piano from Mike Oldfield’s Tubular Bells in The Exorcist and Bernard Herrmann's stabbing strings during the shower scene in Psycho. “You want something that gets into the audience's mind, that starts working on them. That's what it's about.”

Playing those soundtracks for the first time in decades has been less a nostalgia trip than a happy relief. “The first thing I thought was, ‘Thank God these are simple!’” Carpenter confesses. “I don't have the chops for anything more complicated.” As he revisits these pieces nightly, with scenes from corresponding movies projected behind his band, he’s amazed at the reception. “It’s great that people remember this stuff,” he enthuses. “Some of it is from before some audience members were born. But I guess many generations have listened to it, which is wonderful.”

For Carpenter, this surprise second act is simply a bonus to a satisfying career, one that he's enjoying to the fullest. “I never thought about doing something like this before,” he says. “I accidentally fell into it, and it's unbelievable. I get to make music, man! It's a flowing, natural process, and it came at a great time in my life. Now that I'm an old man, I love it.”

John Carpenter performs in the UK in October and November