The War On Drugs were about halfway through their set in the sweaty confines of Corsica Studios last year when band leader Adam Granduciel announced that they were about to play ‘Brothers’, arguably the centrepiece of their stunning last record Slave Ambient. “We usually invite someone from the audience up to play guitar on this one, but I don’t think there’s room,” he continued, glancing around the tiny stage. “Ah, fuck it! Who wants to play guitar?” The guy who cheered loudest was invited up, but his tentative strumming made it quickly apparent that he wasn’t as confident as he first sounded. There was just a trace of consternation in Granduciel’s voice when he told him: “Erm… this is a big show for us, dude.”

Moments later the singer turned back to face the audience. “Hey, that’s what rock’n’roll is all about,” he announced. “It doesn’t matter whether he can play or not. He said he wanted to play and now he’s up here. Fuck it.” No matter. As the band launched into the song the new guy’s tentative strumming became just a drop in their squalling ocean of sound.

Months later, when I remind Granduciel of that moment he recalls it instantly, and with a chuckle: “I think that might have been the last time we’re going to do that! It’s funny because it’s such an easy song to play that normally we’d been lucky. People who wanted to come up and play with the band in front of a lot of people would be pretty confident in their playing ability. We’d never had an issue, but on that night I remember the guy... well, he was left-handed to start with, and we obviously didn’t have any left-handed guitars. The whole thing was pretty amusing. I still give the guy credit for getting up there! He’d definitely had a couple of drinks but he didn’t let not having a left-handed guitar stop him! For me, it’s one of those things that takes the energy of the room to a different place. It changes the mood in a good way, whether or not the person coming up is capable, it’s more about the fact that they wanted to be up there... it’s more about just having a good time. At the end of the day, it’s just rock music!”

The War On Drugs bring their rock music back to the UK this week, and the widely-acknowledged brilliance of Slave Ambient means they’re playing bigger rooms than ever. As well as the pleasure of playing to larger audiences, for someone as obsessive about the way his music sounds as Granduciel there’s also a sonic reward: “When you start playing bigger rooms the music just sounds better. The show at Corsica Studios was awesome but it’s definitely not the best environment for a lot of the War On Drugs music. Also, as a band we really like to play for a while, so hopefully the more people who come and the more our catalogue expands will mean that we can play big rooms and just keep going for two-and-a-half hours! It’s a blessing to play these gorgeous rooms, and to hear our music there is a really big pay-off.”

It’s a pay-off Granduciel’s earned after spending years carefully honing and shaping his music. In 2003 he moved to Philadelphia where he met and started making music with Kurt Vile, who played in an early incarnation of The War On Drugs. To support himself, Granduciel was working for a property management company. His job was to clear out apartments when people, mostly college students, moved on and left detritus in their wake. It was the sort of job that allowed him to concentrate his energy on his music. “There was a lot to do, but there was a lot of down time as well,” he remembers. “In the main office there was a computer on the third floor and I would come in in the morning and work for an hour and then disappear up to the third floor to work on the album art for the first record. I’d come down at like five in the afternoon and they’d be like: “You were here today?” and I’d make up some excuse about what I’d been working on up there. That’s the time when you really find out for yourself the extent to which you’re committed to something. I was working all the time to pay the rent, and also to buy musical equipment and records. I’d work all day and then come home and jam all night. I was playing shows, recording, manipulating sounds, partying and fucking around. We’d go on tour and then get back and have to go to work the next day. It was all because we just loved what we were doing. We put a lot of time into it, and really focused on just being better musicians. I still spend all my time working on new music and just fucking around with sounds. I think it taught me a certain working method that I feel comfortable with and get great results from. It would have been easier to not work as hard as I did all the time, but I feel like it was worth it.”

You can hear the results of that work in the densely-layered music on Slave Ambient, which Granduciel spent weeks and months fine-tuning. He’s been making music this way for so long that things like lyric-writing have become almost instinctive: “A lot of the stuff that I’m really proud of on the record was stuff that just came out in the moment while I was working on the song. Lines that would just pop into my head when I was writing it or recording a vocal. A lot of times I’d go into the studio with just the music and take a couple of hours to improvise and take notes. I got some real gems out of just zoning in to what I was hearing in the music. In ‘I Was There’ there’s a line: “I was there to catch a man, I thought I had him by the hand, I only had him by the glove,” which just came out at the spur of the moment and I still can’t believe I got something like that.”

He raves about seeing his heroes play live, people like Neil Young and Bob Dylan, but he thinks it’s reductive for critics to peg his band as “throwback Americana”. It’s not a throwback, he says, because: “we’re trying to do something new with what we love.”

When the band had a little time off from touring recently, Granduciel predictably couldn’t be kept out of the studio for long. He read Thomas McGuane’s short story collection Gallatin Canyon and cooked his signature chicken soup, but mostly he just recorded and recorded. “I was writing a lot,” he says. “I booked some studio time and went in with the band. It’s nice to have some new stuff to listen to and to put into the set. By the time it comes to record ‘for real’, when we put together the next record, it’ll be nice to have stuff that we’ve been playing live as a band for a while.”

He’s taking his band across Europe this month, and recently he celebrated his birthday with a show at the Melkweg in Amsterdam. He describes it as coming “full circle”, having spent countless hours listening to Spacemen 3’s classic live album Performance, recorded at the Dutch venue in 1988. He’s enjoying the challenge of living up to the music he loves: “In terms of making the records I try to make them as interesting and as much of a work of art as possible, but then live we take those big ideas and turn it into music for a full-fledged rock band.”

Granduciel is a man doing what he loves, on his own terms, which might be what makes his records and his shows so utterly and purely exhilarating. “From the moment I started playing I knew it was something I really, really loved. I never hunted down a record deal. I was really just enjoying what I was doing and it fell into place. I was aware of having to make sure that I was ready for it. I just focused on writing and recording and amazingly it got around to the people who wanted to put it out.” Listening to him talk, I keep thinking back to what he said to that audience in the sweaty confines of Corsica Studios. Hey, this is what rock’n’roll is all about. “Since I was young,” he tells me matter-of-factly, “I’ve always known that all I wanted to do was play guitar my whole life.”