Delta may be the first studio project Mumford & Sons have released in three years, but that doesn't mean the group hasn't been making music during that time. In fact, according to band member Winston Marshall, at least one of the songs on the new album has been in the works since before the group released their last project, 2015's Wilder Mind.

"A song called "Forever," which is the second-to-last song [on Delta] -- I think we originally played it together in 2013, and we'd done various different versions of it," Marshall explained to The Boot in a recent interview. "It wasn't until [producer] Paul Epworth got his hands on it that we finally managed to get a version of it we were happy with."

In fact, it was Epworth's direction and experience in working with artists of vastly different styles that persuaded Mumford & Sons to work with him on Delta, an expansive and ambitious project that pulls influence from a variety of different genres. "This album, for us, is the most varied record we've ever done, and there's songs on it where I hear more neo-classical stuff, on the same record as very pop stuff, rock stuff, dance stuff," Marshall adds, "and I think that he's had the oversight to bring all those things together."

Mumford & Sons -- bandleader Marcus Mumford, keys player Ben Lovett, Marshall on guitar and banjo and bassist Ted Dwane -- went into the studio with Epworth after a two-year period of accumulating songs, hoping that he would be a good fit for the project. "We actually tried a few songs before hitting the jackpot with "Slip Away," which is on the record," Marshall recalls. "When that song came together, it was a eureka moment, where it was clear that it was a good fit."

Wilder Mind had represented a turn away from the acoustic instruments that were previously a staple of Mumford & Sons' music. However, with Delta, the band hoped to reinvigorate their love for those instruments while finding a way to once again reinvent their sound.

"Ben had the idea of, 'Look, how do we use instruments and make them sound like they've not sounded before?'" Marshall goes on to say. "With that in mind, that really triggered something in us and really got us excited. And, coincidentally, without telling Paul, he was onto the same sort of kick. He wanted to do something different with the instruments ...

"There's lots of different moments where you don't realize you're listening to an accordion, or you don't realize you're listening to an acoustic guitar or banjo [on this album]," Marshall shares, "and I'm very proud of that fact."

Courtesy of Sacks & Co.

As they made the album, Mumford & Sons were so captivated by the process of nailing this aspect of the production that it didn't originally occur to them that they would need to figure out how to play these songs live. The band recently announced an extensive, 60-date arena tour, set to kick off on Friday (Nov. 16), the day of Delta's release.

"It's been a challenge, definitely," Marshall says with a laugh of adapting the songs' production style into a live set. Additionally, the stage design of the show will be a more complicated version of an in-the-round performance, with the audience below and above the band as well as on all sides of them.

"It's a whole new level of performance for us," he says, "so we're trying to get our heads around that at the moment, as well as trying to get our heads around playing these songs."

However, the challenges brought on by their new project have served an important purpose for Mumford & Sons: They've renewed the artists' love for their musical style and instruments.

"It's certainly reinvigorated my love for [the banjo]," Marshall adds. "I look at the other guys, and I can see how much they're getting out of those instruments as well. We've been feeling pretty excited, generally."