TORONTO - Irish supergroup U2 doesn't usually open for anyone, but the rock gods will make an exception Thursday when a movie about their career kicks off the Toronto International Film Festival.

The screening of "From The Sky Down" will mark the first time the city's annual movie marathon has launched with a documentary, a choice that co-director Cameron Bailey says hits the right note for the 36th annual fest.

"It's about the biggest band in the world, it's by one of the best documentary filmmakers in the world," Bailey says of the film helmed by Davis Guggenheim, the Oscar-winning director of "An Inconvenient Truth."

"We felt like it would get people going in terms of just charging them up."

There's a delicate alchemy to picking the right film to open a festival.

"We're looking for a film that can play to a fairly general audience," says Bailey, noting that all kinds of film-goers attend opening night so the kick-off movie should have broad appeal.

"You want a movie that has the kind of scale that suits the opening or the closing night position -- a smaller, more intimate film may not work as well as a grander film. And you also want a film that's going to set the tone ... Is it an upbeat film in a year when you want that?"

The Toronto fest has traditionally given the pole position to a homegrown filmmaker -- the campy romance "Score: A Hockey Musical" claimed the berth last September, Paul Gross's war epic "Passchendaele" took it in 2008, Jeremy Podeswa's holocaust drama "Fugitive Pieces" launched it in 2007 and Deepa Mehta's Oscar-nominated period piece "Water" kicked things off in 2005.

It's arguable that Toronto actress-turned-director Sarah Polley gets a better slot than any of them when her much-anticipated romantic drama "Take This Waltz" debuts Saturday at Roy Thomson Hall. It lands on the heavily attended first weekend of the fest and screens at its most prestigious venue.

Polley says she's happy to have scored the gala spot, noting it's just what she had been hoping for.

"I think opening night can be a tough slot," says Polley.

"I think it can be really great for some films and not so great for other films. For us, our ambition was just to have a gala at Toronto and we didn't really care in what context it was so we're really thrilled."

Despite mounds of publicity, there are drawbacks to being the curtain-raiser, says Canadian uber-producer Robert Lantos, whose films have launched Toronto's annual movie marathon several times.

He says any filmmaker seeking foreign sales is better off avoiding the opening slot, noting that it has "become kind of a ghetto" over the years.

"The international distributors and buyers didn't bother showing up for it because it was considered a strictly Canadian event," says Lantos, who expected this year's surprise documentary selection to help stir things up.

"The very people that you want to see your film the most are not there."

He says the real prestige spot is the first weekend when attendance tends to be highest.

"However, it's also the most competitive," he acknowledges.

Friday night galas this year include Brad Pitt's baseball drama "Moneyball," George Clooney's political thriller "The Ides of March," the Kristen Wiig and Jon Hamm comedy "Friends With Kids" and the Jude Law relationship drama "360."

Saturday's stellar lineup includes Polley's relationship study, David Cronenberg's psycho-sexual feature "A Dangerous Method," Lars von Trier's arthouse sensation "Melancholia," Clooney's family drama "The Descendants" and the Ryan Gosling thriller "Drive."

Podeswa says every filmmaker jockeys for the best slot they can, based on the amount of press they're most likely to get and the number of deal-makers available to see the movie.

"There are all kinds of considerations about when people come into town, when they leave town, how big is the venue and how many screenings you're going to get," says Podeswa.

Bailey says such attention to detail can make his job as programmer all the more difficult.

"We have over 250 films in the festival, all of them are looking for what they consider to be the prime position," he notes.

"And there's only one first Saturday night at the festival, there are only two films that play in Roy Thomson Hall that night. A lot of films want that because they feel like that's the best exposure you can possibly get. But the fact is that actually each film has its own best place."

He notes that a lot of filmmakers prefer to screen later in the week when hype over the big-name titles has dissipated, allowing them to break through into the spotlight.

Determining the right day and time a film should have its debut has become a fine science for some filmmakers, says Bailey.

"There are some people who swear that certain cinemas play certain kinds of films better, comedies will play better than dramas, long films will play in some cinemas better than short films and how wide or narrow a theatre is, the angle of the seats, all of those things seem to matter," he says.

"The people who bring movies here year after year, they have opinions about all of these different factors and so we get into very long conversations and negotiations about when and where and how each film should play."

If there's a spot to avoid, veteran director Norman Jewison says it would be the last one.

"If it was me, I wouldn't like to close a festival because I think people are worn out," says Jewison, who adds he's "not a great believer that opening a festival means that you've got the most important film."

"There's too many films in front of you."

Lantos says the best slots depend on the festival.

For the Cannes Film Festival, he prefers to screen towards the end of the run because those are the films that the jury tends to favour.

"The times that my films have won there, like 'Sweet Hereafter' won the Grand Prix and 'Crash' won a jury award, whenever that's happened it's usually been that the films have been late (in the schedule)," he says.

"And I think you could say the same in any festival that has a jury. A jury tends to be most critical at the beginning and then after they see a dozen films that they're not thrilled about they are more enthusiastic about one that they really like and tend to remember it best."

No slot can help a bad film, says Jewison.

"If a film works with the audience first that's what's important," he says.

Several Toronto openers have failed to springboard to box office glory. Last year's kickoff, "Score: A Hockey Musical" disappeared quickly after mixed reviews while 2009's Brit biopic "Creation" drew a lukewarm reception and collected less than $900,000 worldwide.

Podeswa says scoring the opening slot for his 2007 drama "Fugitive Pieces" helped his film overall, noting it secured him a U.S. distribution deal.

"It's always hard to say what exactly consequentially relates to what or what's the causal thing but our film got picked up by (Samuel) Goldwyn and we got distribution and that was all good," he says.

"And then the opening night was really exciting and fantastic. In a way, for me, that's an end in itself -- it was so nice for the cast and the crew and my parents, you know. For all the personal reasons it was so great to open the festival."

Bailey admits he continues to rethink his choice right up until the night of the screening, and even as it plays in front of an audience.

"We've had great opening nights, we've had opening nights where the audience hasn't been as thrilled as we were," he admits.

"We're making these decisions on our own as programmers because we spent a long time doing this, we have experience in it but we also hope that we understand our audience well enough to know what's going to interest them. But we're not right 100 per cent of the time and so we have to evaluate that after the fact and see, 'Well, did our choice really match what they were interested in seeing that year?'

"And if not then we try to do better next year."

The Toronto International Film Festival runs Thursday through Sept. 18.