‘I don’t have many friends. Do they just want to be your friend because you’re Janet Jackson?’

Early in his sprawling four-part chronicling of the life and times of Janet Jackson, director Benjamin Hirsch asks Jackson why she agreed to participate in the film.

“It was something that needs to be done,” she says. “You’ve had somebody else write this authorised biography.”

Janet Jackson (Sky Documentaries, 9pm) may carry the seal of approval of the youngest member of the Jackson showbusiness dynasty. But episodes one and two never feel quite as authoritative as they should – perhaps because Jackson seems unsure how much of herself she wants to give away.

“Ben, I don’t want to talk about this any more,” she says during an interview about her short-lived first marriage, to drug-addicted pop star James DeBarge. And the harshest thing she has to say about her late brother Michael – whose reputation now lies in ruins following accusations of paedophilia – is that he cordoned himself off from the rest of the family as mega-stardom beckoned.

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She and “Mike” had, she explains, “started going their separate ways” when Thriller came out in 1983. Of the claims of child abuse against Jackson, she says, “They build you up and then, once you’re there, they’re so quick to tear you down.”

And yet for all her reticence a portrait emerges of someone who had stardom thrust upon them rather than actively seek it out. Forced into the family business of pop music by her ruthless and relentless father Joe, Janet was on stage before her tenth birthday and recording a debut album at 18. There were also acting roles, in the 1970s sitcom Good Times and later in Fame. What she really wanted, however, was to go to college and study business law.

“What parent doesn’t want you to go to college?,” she wonders, referring to Joe’s instance she follow brothers Michael, Tito, Jackie, Marlin and Jermaine into music. “He said. ‘no you’re going to sing’.”

The film clocks in at nearly four hours in total and is part of a vogue for long-form documentaries in which the key word is "long". Showtime's new Bill Cosby doc is similarly gargantuan as was Peter Bogdanovich's 2007 love letter to Tom Petty.

But if the film is forensic in exploring Jackson's early childhood in Gary, Indiana and her later upbringing in the upmarket Los Angeles neighbourhood to which the family moved after the success of the Jackson 5, such comprehensiveness robs the story of momentum.

Many of the biggest controversies in Jackson's life are relegated to later episodes. The paedophilia accusations against Michael Jackson are not raised until instalment three (airing Tuesday night). And we have to wait even longer before Hirsch brings up "Nipplegate" , in which Jackson was forced to carry the can for Justin Timberlake ripping her top at the 2004 Super Bowl.

This, then, is not an authoritative portrait of Jackson as woman and artist. It is, rather, a simmering rumination on fame and the toll it extracts. And yet one that leaves little doubt that the singer has paid a price for her celebrity. She says as much herself in an archive interview, when asked if there is a downside to being a member of the first family of American pop.

“The disadvantage is choosing a friend,” she says. “You don’t know if they’re really your friend. Is it who you are or what you are? Do they just want to be your friend because you’re Janet Jackson or Michael Jackson’s sister? I don’t have very many friends.”