'Coz this is THRILLER, thriller night, and no one's gonna save you from the beast about to strike. You know it's THRILLER!" Singing/screaming these lyrics is how I decide to wake my flatmate Fionn on Wednesday morning at 2.30am. He is not amused. Fionn is the only one I could convince to get up at all hours, trek out to the Liffey Valley cinema, and check out the 4am screening of Michael Jackson's This Is It – a load of rehearsal footage that preceded the London O2 shows that never were, edited together to make a feature-length film. It's showing at stupid o'clock to coincide with the time in LA that the film premieres.


We're in the taxi on the way there. The taxi driver thinks we're nuts. "I wonder if this is normally a 24-hour cinema," Fionn muses. "Did someone tell you it was?" the taxi driver scoffs into the rearview mirror, clearly coming to the conclusion that we're the victims of some practical joke.


The first person we see on arrival is a young guy dressed as Off The Wall-era Jacko, complete with a giant cardboard poster of the album cover strapped to his back. The mixture of Michael Jackson fans is, shall we say, eclectic. Some have clearly stayed up all night and are now devouring the drinks at the reception (cans of Budweiser and Miller, wine) laid on by Sony and the Vue cinema. Others look just as delirious as myself and Fionn. There are young kids dressed up, complete with black hats and white gloves, middle-aged women smoking outside, teenage girls, and loads of guys in their 20s sporting This Is It tee-shirts.


Fionn grabs a glass of wine. I go for coffee. My body is extremely confused. Usually if I'm up at this hour, I'm head first in a kebab, not in a shopping-centre cinema in the 'burbs. A DJ is blaring Michael Jackson hits as the fans begin to queue up on a red carpet splattered with glitter that leads up to the screening door. Eventually, the noise, heat, music, and visual stimulation becomes too much for my delirious brain. I think I'm going to puke. After getting some air outside and downing a bottle of Powerade, it's time for kick off.


Inside the cinema, DJ Ian Dempsey announces best-dressed winners (Off The Wall guy chooses a Bond-film box set, one of the kids goes for a Sony camera) and a jacket sported by Jackson during the Bad years currently residing in a gallery in Newbridge is moved aside. The film starts, beginning with interviews with the tour's dancers. Those poor dancers. Those 50 concerts with the King of Pop was going to be the pinnacle of their careers. It would have been impossible to beat. But they never got to realise that dream.


The film itself, in spite of my cynicism at the endless cashing-in of Michael Jackson's legacy, is compelling. It's incredibly simple, featuring only a collection of footage filmed at the rehearsal space in the Staples Centre. One imagines that there were hundreds of hours filmed, many of which featured Jackson in less-than-good form, but none of that is here. This is a different version of the truth. This is Jackson as a complete pro. His voice is astounding. His dancing, even though he is painfully skinny, and at times hunched and stiff, is phenomenal. The backing dancers are brilliant, trained and practised to within an inch of their baggy-tracksuit-bottomed lives. But Jackson's dancing isn't about training – it's instinct, it's his music moving through him and demanding him to move.


At times, he's a little loopy, and throughout he repeats the phrases "God bless you" and "I love you", but the message is clear: this is a guy who wanted to do these gigs. There is no coercion. He is intrinsically involved with the musicians and dancers and other vocalists, changing arrangements, tweaking sounds, all the while cheered on by an almost evangelical crew consistently astounded by his presence. If those shows had gone ahead, they probably would have been landmarks in popular music. What a pity he didn't last to show the world that he was still all there – at least in creative and performing terms, whatever about mental or physical ones.


Throughout the film, the crowd is silent, occasionally clapping after a particularly impressive section, bopping along seated to the catchier numbers, with a steady stream of "oh wows" coming from the ladies next to me. Afterwards, chatting to fans, the same word comes up again and again: "perfectionist".


A bunch of teenagers with their mum emerge delighted. One, Pamela Keegan from Crumlin, is almost lost for words. "It was amazing. It was unbelievable. It was kind of like seeming him in real life." Two other young folks wander out astounded by the movie. Graham from Knocklyon says, "It was great. I'm a big fan. The guy was a perfectionist." Aoife from Skerries was similarly impressed: "The show would have been unbelievable. He was an absolute perfectionist by the looks of things. The people working with him seemed to be the best in the business. The dancers were unreal, the musicians were brilliant."


This Is It shows us the Jacko we never knew: the industry veteran, the musician's musician, the dancer's dancer, the pro, the perfectionist. On the way back home, after 6am, the bizarreness and surreal nature of the experience and tiredness of my brain struggling to figure out what the hell is going on evaporates. All Fionn and I can talk about is Jacko himself as an entertainer. This new unseen side, that in some ways (although not completely) clouds out the endless wranglings over his death, the craziness, the rumours, the surgery, the dodgy relationships with kids. This Is It shows up what we forget about his legacy.


Dude, this was a real thriller.


'This Is It' is in cinemas now