Spike Lee worked with Michael Jackson and considered him a friend, but the director says even he learned a lot combing through footage of the icon for a planned documentary about the singer's Bad album.

Lee calls it a "treasure chest of findings."

"We have footage in this documentary that no one's ever seen, stuff that Michael shot himself, behind-the-scenes stuff," he said in an interview Monday. "We had complete access to the vaults of Michael Jackson... He wrote 60 demos for the Bad record. Only 11 made it. So we got to hear a lot of that stuff, too, so it was just a great experience."

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Michael Jackson, who died in 2009, is shown performing onstage circa 1987, the year he released his hit album Bad. (Dave Hogan/Getty Images)

He added: "You don't have to be a Michael Jackson-head to enjoy this."

Lee's documentary will be part of a flood of material to celebrate the 25th anniversary of the Bad album, Jackson's follow-up to Thriller that included hits like the title track, Smooth Criminal, The Way You Make Me Feel and more. The album is being rereleased Sept.18 with additional tracks, a DVD and other bonus material; Lee's film is due to come out later this year, but no date has been set.

Jackson's personal side

Besides Jackson's artistry, Lee said the documentary will show a more personal side of the late legend.

"He had a great sense of humour, and he was funny — so you'll see a lot of that stuff," he said.

Lee interviewed people ranging from Kanye West to Mariah Carey to Sheryl Crow, who was Jackson's background singer on the Bad tour.

"We really divided it into two things: Artists today who were influenced by Michael, and then people who worked side by side — musicians, songwriters, technicians, engineers, people at the label, who were all committed to Michael, to the follow-up to the biggest record of all time, which still is Thriller."

Besides the documentary, Lee also plans to hold his now annual birthday tribute to Jackson in Brooklyn, N.Y, on Aug. 25; Jackson would have turned 54 this year.

"This year we're going to focus on the Bad album, we're going to focus on the 25th anniversary, but at the same time we're going to play his full catalogue, including the Jackson 5 and the Jacksons," he said. "It's going to be even bigger and better this year."