There’s something about the Doors and their music that seems to rile people up. Filmmaker Tom DiCillo discovered this as he was feverishly trying to complete his documentary about the group, “When You’re Strange,” in time for last year’s Sundance Film Festival.

“The Doors are enveloped in a cloak of legend so thick sometimes it is impossible to penetrate,” blogged Tom DiCillo at the time. “People have deeply personal and passionate convictions regarding the ‘Truth’ about the band.”

Perhaps that need for “truth” began when the group’s mercurial lead singer, Jim Morrison, was found dead in the bathtub of his Paris apartment in 1971 at age 27. No autopsy was performed.

When DiCillo’s film, now getting a limited release April 9, was screened at the Park City showcase in 2009 as a work in progress, some critics and distributors stormed out of the theater, convinced the filmmakers had used an actor to play Morrison in the film’s opening sequence.

What they actually saw were outtakes of a bearded (but not bloated) Morrison from a 51-minute short called “Hwy: An American Pastoral” that Morrison, a UCLA film school graduate, had made with Paul Ferrara in 1969. DiCillo discovered the original 35 millimeter master, “literally right before we finished,” he tells Variety.

The footage, which DiCillo uses to frame his 89-minute film, is one feature that sets “When You’re Strange” apart from previous documents of the Doors, one of the most chronicled bands to emerge from the late-’60s/early-’70s pop-rock renaissance.

“Every new generation seems to have to go through its Doors rite of passage,” says John Densmore, the group’s drummer, and one of three surviving members of the group — along with guitarist Robby Krieger and keyboardist Ray Manzarek. The film was made with the cooperation of the band, which supplied most of the footage, as well as the families of Morrison and that of his late girlfriend Pamela Courson, who also died at age 27.

Reviews out of Sundance 2009 were mixed (this paper was particularly scathing). But since then, the writer-director’s wall-to-wall narration has been pared down considerably, with actor Johnny Depp recruited to do the voiceover. “It’s the inflections,” explains Densmore about Depp’s contribution, “it’s so personable. He’s an icon like Jim, so he brings something that’s closer to Jim.”

“When You’re Strange” dispenses with the usual talking heads format that characterizes so many VH1-styled music docs — or what producer and Rhino Entertainment exec John Beug refers to as “a bunch of old pirates talking about the good old days” — in favor of allowing the footage, and the music, to speak for itself.

That music — alternately rough-and-tumble rock, jazz-inspired improvisation and Brechtian, existential musings — is so exotic that no group could be mistaken for the foursome that formed in Venice, Calif., in 1965, and released six studio albums by the time of Morrison’s death.

Still, the group remains one of those phenomena that rises, Phoenix-like, every decade or so. When “Apocalypse Now” was released in 1979, the use of the Doors’ “The End” helped triple the group’s catalog sales, according to Jeff Jampol, the Doors manager. Similar sales spikes occurred when the sensationalistic Doors bio, “No One Here Gets Out Alive,” was published in 1980 and then in 1991 when Oliver Stone’s “The Doors” hit theaters.

The 10-12 market theatrical engagement of “Strange,” produced by Dick Wolf (of “Law and Order” fame) in association with Rhino, will be followed by a May 26 airing on PBS as part of its American Masters series, with a DVD release soon thereafter. (Along with Wolf and Beug, Jampol and Peter Jankowski are credited producers.)

Bruce Botnick, who engineered the Doors albums and produced “L.A. Woman,” supervised and mixed all the music for the doc.

According to the RIAA, the Doors have sold more than 32.5 million albums in the U.S., much of them after the group finally called it quits in 1973.

Beug acknowledges that music docs are a tough sell. “The major screens aren’t particularly interested,” he says. But the filmmakers are hoping to recoup their $2 million investment, and then some, via ancillary sales.

A soundtrack, which is still being compiled, will include previously released songs from the Doors, readings of Morrison’s poetry by Depp, and live recordings from scenes featured in the film, such as their set at the Isle of Wight in 1970 and a TV appearance on “The Ed Sullivan Show.”

Regardless of how the doc performs, the filmmakers can take some satisfaction in having produced a movie that all those associated with the Doors, who have had their differences, are satisfied with.

“I can’t quite put my finger on what Tom DiCillo did,” Densmore says, “but he somehow found more depth and magic. You do get a feel for the turmoil of the times. It was a time of flower power and maybe we were the shadow side of the undeclared war in Vietnam and writing darker stuff, and it seemed to last longer.”

Adds DiCillo: “I hope (the doc) allows people to see them as close as possible to what they were. Strip the myth away, strip all the bullshit away — the drugs, the sex and alcohol (associated with) Morrison, that certainly was a part of him — and really look at (the group) and what they accomplished.”