"One of Osama Bin Laden's wives revealed to the CIA that Osama Bin Laden had an obsession with Whitney Houston," Kevin Macdonald, director of Whitney, recounts bemusedly, "and was planning, or at least idly thinking about and talking to his wife about sending an assassination squad to America to kill Bobby Brown." That's only one of the stories that didn't make the final cut of Macdonald's documentary about the late, great Whitney Houston; and if that isn't the litmus test for a life that is overflowing with too much complexity and intrigue, I don't know what is. How surprising then to hear that the director, known for The Last King Of Scotland and Touching The Void, initially had no interest in exploring the life of one of the best-selling artists of all time.

"I did wonder what there was left to say about Whitney Houston," says Macdonald. "But then I met Whitney Houston's agent, a woman called Nicole David, and she said Whitney was my favourite client I ever had' - and Nicole discovered Brad Pitt, so that's saying quite a lot! - but she said "I don't think I ever really knew her, and I don't think anyone understood her. And I think you should make a film to try to understand her.' And there was something about that invitation that was so intriguing. Also the idea of trying to make a film about someone who so many people dismiss now, and who I had partially dismissed, was an interesting challenge."

Fans who grew up loving Houston's music may never have dismissed her, but it's true that particularly towards the end of her life, much of the world had. Her story had become a tabloid narrative of a fallen icon, swept up in drug addiction and dysfunctional relationships. But Macdonald received unprecedented access to Houston's family and friends, and through interviewing them, essentially became a therapist for the family's long-held secrets. These included Houston's introduction to drugs as a teen by her elder brothers, her bisexuality and relationship with Robyn Crawford, and shocking allegations about Houston's experience of childhood sexual abuse by her cousin, Dee Dee Warwick.

It was through these interviews that Macdonald's film began to emerge - but he is also mindful of external factors beyond the family that destabilised Houston's sense of self. "The racial element underlies so much of her life story, and surprised me," muses Macdonald. "She was an outcast in her neighbourhood and was bullied at school for being light-skinned and dressing too prettily, dressing 'too white.' Then later when she was successful, Al Sharpton had campaigns calling her 'Whitey Houston' and saying she was selling out. She was booed at the Soul Train Awards, but also gave a rendition of 'The Star Spangled Banner' that really touched Black Americans. I think she was very confused about her racial identity. It wasn't simple for her. The fact that she was one of the few Black stars of the MTV generation and made it big made Black audiences suspicious of her. But then she also appeared in The Bodyguard, which was a film where race is never mentioned, where she gets a love story with a white man. Even today, that would be notable."

A small but fascinating detail shared in Whitney is that at the height of her fame, the singer would visit with Michael Jackson, and the two would simply sit together, not speaking.

"One of the things that fascinates me that the film doesn't really dwell on is that the three African-American stars of that era, and the biggest stars of the era overall, maybe alongside Madonna - Michael Jackson, Prince, Whitney - all ended in remarkably similar ways," muses Macdonald. "They ended up dying young, with drugs involved, having isolated themselves and living somewhat in a fantasy world. Why is that? I don't know the full answer, but I think part of it is that they didn't have peers, and didn't know what to do with their enormous fame."

Macdonald's film does feel tragic, recounting Houston's addiction, her loneliness, and her toxic relationship with Bobby Brown. Though he cut the section from the film's final cut. Macdonald admits that he became morbidly intrigued by the circumstances of her death. "Mary Jones, [Houston's assistant] was the woman who found Whitney's body, and she said to me 'When I went into the hotel room and found the body, there was around six inches of water on the floor in the room - but when I went into the bathroom, the taps were off. But Whitney was floating in a bath that was full to the brim. So who turned the taps off?' So I interviewed everyone I could who had been there," Macdonald says, "and none of their stories matched. So definitely something fishy was going on. I don't think there was foul play, but I think what happened was that somebody brought drugs up to her - she asked her assistant to go out and get her a cupcake - then someone arrived with drugs, and she took drugs in the bath, collapsed, and drowned when someone else was in the other room. I think they then saw the water coming out, went in, saw her and panicked - because they had given her the drugs, presumably. They switched the taps off, and they left. That's my best guess from what I learned. I had a twenty minute section about it but it was so complicated and didn't come to any conclusion."

The film may not offer a conclusion or an easy answer, but it does offer a reintroduction; a reminder of how and beautiful and powerful Whitney Houston's music was, and how her musical legacy lives on. That may be better.