One Direction's Liam Payne: It is hard to see our parents so upset in new documentary

Zayn Malik, Liam Payne, Harry Styles, Niall Horan and Louis Tomlinson in TriStar Pictures' One Direction: This Is Us. (© 2013 TriStar Pictures. All Rights Reserved.)

One Direction star Liam Payne said it was "difficult to watch" the boyband's new film because it showed how upset their parents were when they left home to tour the world.

The singer and bandmates Harry Styles, Zayn Malik, Niall Horan and Louis Tomlinson will be greeted by thousands of fans in Leicester Square tonight ahead of the premiere of One Direction: This Is Us.

One scene in the film, which was made by Super Size Me director Morgan Spurlock and follows the group's life on the road during their rise to worldwide fame, features Payne's father talking about how much he misses his son.

The star told BBC Breakfast: "For me that was a bit difficult to watch. He's always so happy when I see him and happy to have me home; to actually see what he's like when I'm not there was a bit mad."

Bandmate Tomlinson agreed it was "tough" for their parents.

He said: "You know a lot of families anticipate that once you get to 18 you'll go to university and mothers know that that's the day whereas for us it kind of just happened out of the blue and our mums just had to wave goodbye to us on the X Factor so I think it was a lot for them to take in at first."

Fans slept out in Leicester Square overnight in a bid to get a prime spot ahead of the band's arrival on the red carpet, though organisers are encouraging people to stay away and watch from home.

They said: "Event staff on the ground, the Metropolitan Police and Westminster City Council will be working together to ensure the safety of fans, guests and staff on the day. We would like to remind fans that the red carpet is being streamed live all over the world, so they can get the best views from the comfort of their own homes."

The boyband, who were brought together on The X Factor, have topped the charts on both sides of the Atlantic, becoming the first British act to top the Billboard chart with their debut album.

Tomlinson said one advantage of making the film was it helped them remember everything that had happened to them in the last few years.

He said: "Because so much is happening to us, you kind of just lose it all.

"It's great just to sit back and watch it back and remember and relive those moments."

Styles said the film would allow fans to learn more about them, adding: "We just want to show what we're about and with social media and stuff there's only so much you can get across in terms of your personality."

Asked why there was no sex or drugs in the film, he joked: "This is a family press conference."

And Tomlinson said: "We like to think we're rock 'n' roll but we're not really."

Styles said they did not cut many scenes from the film, adding that it "was more putting in stuff we remembered we really wanted".

But Tomlinson said it was difficult to put everything in because "you've got five people's personalities to get across so it's actually quite hard".

Spurlock said fans would enjoy seeing how "normal" the band are.

He said: "There is no air of superiority, there is no air of success that permeates this movie.

"You see five guys who are the same five guys they were three years ago."

PA