The previously unrevealed machinations of U2 have been laid bare in a documentary about the making of the landmark album 'Achtung Baby'.

'From the Sky Down', which premiered at the Toronto Film Festival earlier this week -- the first time a documentary has opened the event in its 36-year history -- is "excruciating" to watch, admitted Bono.

"I don't think really it's a film about our band in as much as it's a film about the creative process. If you're interested in that, you're going to be interested in this film I think, but I find it excruciating," he said

The documentary will be broadcast on US television in late October and will then be included as a DVD with deluxe editions of 'Achtung Baby', due to be re-released near the end of the year.

Bono was joined on the red carpet by bandmate The Edge who said it was shocking to realise how close the band came to splitting.

The film, directed by Oscar-winner Davis Guggenheim, shows the band travelling to Berlin.

Tensions emerge as they struggle to reinvent themselves. However, their experimentalism ultimately pays off with the success of 'Achtung Baby'.