A documentary on Beach Boys cofounder Brian Wilson is nearing completion, producers tell Variety, after nearly three years of under-the-radar production that has involved collecting about 90 hours of fresh footage of the music legend talking about his life or working in the studio. Bruce Springsteen, Elton John and Nick Jonas are among the famous fans who’ve filmed their own interviews for the still-untitled doc, which is expected to be completed and ready to show to potential distributors in January.

The film is being directed by Brent Wilson, who first met Wilson in the course of doing interviews for his documentary on doo-wop, “Streetlight Harmonies.” Acting as producer and financier is Tim Headington, who’s been a producer or executive producer on “Jersey Boys,” “Hugo,” “Argo” and “World War Z”; joining him as producer is former music biz exec Theresa Steele. Wilson’s longtime manager Jean Sievers is co-producing, and executive producers include Wilson and his wife Melinda Wilson as well as Rolling Stone editor Jason Fine.

It won’t be a linear, all-consuming documentary or one with a lot of outside talking heads, says director Brent Wilson (no relation to the star). “Seventy-five percent of what is in the film is Brian,” Wilson estimates, “and then we were really selective about who we wanted to appear in the film, using people like Springsteen, Elton and Jim James (of My Morning Jacket) to reinforce the themes.”

Wilson’s life has been explored on film before, in the biopic “Love and Mercy,” the Don Was-directed documentary “I Just Wasn’t Made for These Times” and several Beach Boys documentaries or TV movies. But none were “definitive,” says the new film’s director, and certainly none were up to date. “We all really felt that Brian is living this really remarkable third act in his life, that as far as I can think of very few artists get to do. I was intrigued by the fact that here Brian was doing something at the age of 75 that he didn’t have the courage to do at 25, which was touring.”

Much of the doc involves the legendary musician sharing memories while driving around with Rolling Stone’s Fine for five or six hours a day during filming, visiting his childhood haunts in Hawthorne, checking out old photo shoot locales in Malibu, or returning to the studios where Beach Boys classics were created. Brent Wilson had planned to do all the interviews with Brian Wilson for the film himself, but a change of plans was in order after he found out what hundreds of interviewers before him already realized — that the man universally recognized as one of the great geniuses of 20th century pop is also one of the toughest interviews in the business.

“I had a much more traditional documentary in mind, where we’re going to interview Brian and have Brian tell his own story — very naive on my part,” laughs Wilson. “And I did the first 20-minute interview, and it just went terribly. And I did a second interview, and itjust went terribly. And I knew that I didn’t have a film. He’s been interviewed a million times since he was 19 years old and been asked a million questions the same way, and he hates being interviewed. He just doesn’t like cameras. He gets so nervous around them. I see these documentaries and these interviews with Brian, and he’s always so uncomfortable. And you can tell he’s giving the pat answers. Never rude, because Brian doesn’t have a rude bone in his body, but he’s just going to say whatever he thinks is going to get him done the quickest. It reminds me of going to the doctor or something: ‘Just give me the shot and get it over with.’”

Part of that has to deal with Wilson’s well-chronicled decades of psychological problems, as well. But as anyone who’s done or read a lot of Wilson interviews also knows, occasionally he blurts out a startlingly rich, spontaneous insight or important memory you’ve never heard before. How to get at more of those? Sievers recommended that Wilson talk with Fine, who had done a number of interviews with the subject before, most memorably a Rolling Stone article titled “Better Days” that had him driving around L.A. with the star for days — a successful experiment they ultimately replicated for the film.

“So we rented a car and we rigged it up, kind of ‘Carpool Karaoke’ style, with 4K cameras and microphones in the car, so that they didn’t have to wear a lavalier mic and didn’t have operators in the car,” Wilson says. “Jason and Brian would just drive around for six, seven, eight hours a day, for weeks, trying to get Brian to open up and talk freely. And it worked brilliantly, because I’ve never seen Brian in any interview or any documentary where he’s been this honest — I mean, just brutally honest sometimes.” (A more traditional documentary team set up shop with Wilson in his favorite hangout, a deli.)

The filming m.o. had a practical purpose, in making a notoriously uncomfortable interview subject comfortable, but it also had a symbolic one. “I also really liked the idea of using kind of the car as a metaphor for telling Brian’s story,” Wilson says. “Those car songs are not about cars. Those songs were about freedom. Bruce has a great quote about how it was really Brian Wilson and Chuck Berry who created the sound of freedom.”

The handful of outside voices interviewed for the film each served an individual purpose, the director says. “Nick Jonas started out as a young teenage performer just like Brian and is trying to make that transition into adulthood. Jim James, who’s very much an indie god, I knew would be able to appreciate what it is that Brian does in the studio. And then Elton is someone who I think understands as much as anybody else besides Brian what it is to create something that sounds like a pop song but is really something else.” Others appearing on screen include the Foo Fighters’ Taylor Hawkins, Gustavo Dudamel, Jakob Dylan and Bob Gaudio.

“And then Bruce Springsteen was at the top of the list,” says the director, “not because he’s Bruce Springsteen, but because, as everybody knows, Bruce has suffered from tremendous depression. And the parallels between Bruce Springsteen and Brian Wilson are incredible. Bruce was really 10 miles in from the Jersey Shore, even though he kind of made up that Jersey Shore scene. Brian created that West Coast beach sound, but of course everybody knows he doesn’t surf, and Hawthorne, even though it’s only five miles from the beach, it’s a thousand miles from the beach. And they both had situations with fathers, when they grew up… They’re the same guy. They really are!”

Besides all the “carpool” footage, Wilson had the other Wilson go into the studio for portions of the doc, albeit mostly to record cover songs, to alleviate the pressure the musician feels of trying to live up to “God Only Knows.” Among the highlights, Wilson says, is “a song called ‘Honeycomb’ that he loves; it’s a Jimmie Rogers song from the ’50s and he does a completely crazy, Brian Wilson version of it.”

Brent Wilson and his producers plan to wrap up a final edit of the project in January and begin efforts to sell it to a distributor at that time.