Michael Jackson's biopic promises to be a pretty long film -- that's what the film's key producer is saying anyway, and it's gonna be chock-full of all his greatest hits.

Graham King made a surprise appearance at Lionsgate's CinemaCon panel Wednesday, where he talked about the forthcoming "Michael" movie that chronicles the late singer's life ... and they even played a first-look trailer at the event.

The footage for that has yet to surface publicly, but word is ... it looks pretty dang good.

Now, in terms of details about the flick -- Graham says it'll be a lengthy feature, as they wanted to do Michael's life full justice ... and he says in order to do that, they needed a long runtime, although he doesn't say exactly how long it'll be.

In any case ... Graham went on to say that there'll be a lot of MJ music in this one -- with over 30 songs from Michael's catalog, so fans can pretty much expect to hear all the hits.

He adds ... "Behind the unrelenting scrutiny and the accusations and the blinding media spotlight, he was simply a man. A man who lived a very complicated life. The movie will get into all of it."

Per THR, the sneak peek kicked off with the Jackson 5 performing their global hit, "ABC," with a closer look at Nia Long's take on Katherine Jackson, the musical family's matriarch. Of course, it was Jaafar Jackson who stole the show ... as he reportedly bore a striking resemblance to his late uncle in the clip.

The trailer reportedly did not indicate if the film will touch on the child molestation allegations against Michael or his subsequent trial -- but the producers of the film say this will be touched on in some capacity ... although it remains to be seen exactly how.