As the mecca of country music, the Ryman Auditorium in Nashville, formerly home of the Grand Ole Opry, has hosted all the greats, from Patsy Cline and Johnny Cash to Dolly Parton and Elvis Presley. But few have created quite such a frisson of excitement as the silver-haired figure standing nervously backstage, wondering whether he should have worn a tie to go with his suit and check shirt. “I feel pretty underdressed,” he confesses as a woman barks an order for three cheeseburgers with fries into a walkie-talkie with an urgency that suggests the world may stop turning if they don’t arrive immediately. Everyone is a little bit nervous because, as the Ryman’s marketing director puts it: “It’s not every bluegrass band [that] has a movie star in it.”

The bluegrass band appearing tonight is the Steep Canyon Rangers, who, as their guest banjo player puts it, have taken time out from ranging steep canyons to become his backing group. Steve Martin is, of course, better known as the star of hilarious films such as The Jerk, Roxanne and Planes, Trains and Automobiles, and less hilarious ones such as The Pink Panther and Cheaper by the Dozen. But tonight he has a different role: playing bluegrass on the banjo. And it’s a job he takes very seriously indeed.

Not that the evening is entirely joke-free; far from it. The quip about ranging steep canyons is one of many gags between songs, and it would be unfair to steal any more material as he’s coming to London to play his first headline show as a musician. But the joke is firmly on those who imagine this is yet another example of an egotistical celebrity (mentioning no Russell Crowes or Keanu Reeveses) making an ill-advised foray into music; Martin has even shared a Grammy for his appearance on an album by the bluegrass legend Earl Scruggs.

In fact, Martin has been playing the banjo for almost 50 years, and is accomplished enough to have won the approval of such banjo virtuosos as Scruggs (best known for the Beverly Hillbillies theme), Bela Fleck and Tony Trischka.

In his dressing room backstage at the Ryman, a visibly nervous Martin signs posters and nibbles at a vegetarian snack of sugar snaps and tofu. “Don’t tell anybody I have tofu,” he demands, deadpan, before explaining how he came to be performing at the home of country music. He’s aware British audiences may not know he used to employ a banjo in his early-1970s stand-up comedy routines, “though I played it on the Muppet Show once”, he adds. His love affair with the banjo, however, began earlier than that, when he was growing up in the early 1960s in Southern California. Surely he should have been smoking pot and listening to Bob Dylan? “No, not yet,” he replies.

Martin’s influence was the early-1960s folk craze, headlined by the group the Kingston Trio. “They wore striped short-sleeved shirts and pants called ‘peggers’ and were very clean cut. They played it like this,” says Martin, grabbing a banjo to demonstrate different playing styles. Until then he had been listening to “rock’ n’roll” and had never played an instrument. But the banjo immediately struck a chord. “I just loved the sound,” he says, “and I can’t really tell you why. It’s hard to express why, except that whenever I heard a song on the radio that had a banjo on it, I would find myself going, ‘What’s that?’”

At 17, he bought his first banjo and taught himself to play from a book by the folk legend Pete Seeger. “I had no musical background, and that was the only book. It was called How to Play the Five-String Banjo, and it showed me where to put my fingers down for the chords, though at first I couldn’t tell the difference between one chord and another. He had some pretty fundamental stuff in there, but it was really helpful. In fact, I still use some of the things I learnt in that book today, like how to play in three-quarter time.”

When he began his career in stand-up, he used his banjo as a prop, playing bluegrass songs purely to pad out his show. “When I first started I would play serious songs, but when the comedy act started taking off, I just knew that the audience weren’t there to have me play serious banjo, so I worked out some comedy routines.” Among these was a spoof bluegrass tune, Ramblin’ Guy, for which he would divide the audience into absurd configurations (two-fifths and three-fifths; two-sevenths and five-sevenths) and encourage them to sing along... in Chinese.

There’s none of that on his current tour — although there are enough gags to make the evening a treat, even for those who find bluegrass a somewhat samey genre of music. But he’s entirely serious about the music. He wrote all but one of the 16 songs on his album The Crow: New Songs for the Five-String Banjo, recorded with a host of bluegrass greats. It includes some songs he composed 40 years ago. Since then, bluegrass music has enjoyed a revival thanks to the O Brother, Where Art Thou? film soundtrack, which sold more than 7m copies and helped turn Alison Krauss into a superstar.

Martin is modest about his talents — “I would never call myself a virtuoso” — but the banjo genius Bela Fleck, who is arguably the world’s most accomplished player, is full of praise. “Steve is a really good banjo player,” he confirms. “He’s not trying to be a technical virtuoso. He writes these simple soulful tunes that he plays very well. He has a beautiful tone and a great feel and a lot of imagination. His record is full of beautiful tunes.” The instrument’s best-known exponent, the 85-year-old Earl Scruggs, gave his own seal of approval by joining Martin on stage at the Ryman to perform his classic tune Foggy Mountain Breakdown. As for Martin himself, he says: “I’m not making any extravagant claims for myself and I’m not claiming to be a virtuoso. I’m either an amateur professional, or a professional amateur — I’m not sure which.”

Although he abandoned banjo playing in public more than 30 years ago, he has never stopped playing for fun and has been known to pick up his banjo between takes on film sets. He began taking it more seriously when Scruggs invited him to appear on a 2005 album featuring “family and friends”, but felt his performance had been “a little bit rusty” and decided to practise more. “So what I did is a little trick: I put a banjo in every room in the house — the living room, the dining room and the bedroom — so wherever I was, I could pick up a banjo. And then I started writing some tunes again.”

Another highly acclaimed banjo player, Tony Trischka, then invited him to play on his Double Banjo Bluegrass Spectacular album last year. “And that’s where it really started. I said, ‘There’s no point in me playing just a traditional banjo song, because there’s 10 guys who could play it better than I could — but I have some songs of my own.’ So he came over and listened to them and said, ‘Okay, let’s do that, that’s good.’” They recorded Martin’s own composition The Crow, which prompted the actor to do an album of his own, initially as a collaboration, but his songwriting was so prolific that he had enough songs for a complete album.

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