One Direction have joked their upcoming movie will show their "private parts".

The chart-topping band - comprising Niall Horan, Harry Styles, Zayn Malik, Louis Tomlinson and Liam Payne - are starring in upcoming 3D documentary film 'This Is Us' which captures their meteoric rise to fame since appearing on The X Factor in 2010.

The heartthrobs made an innuendo about being surrounded by cameras the whole time.

Styles revealed: "It's always going to be scary when you're allowing someone to come in and film everything you're doing because it's …" before Payne cheekily chimed: "Your private parts."

Tomlinson added: "And they are meant to be private," before Styles smiled: "Because they're the parts that are supposed to be covered. But they're not."

Tomlinson jokingly explained: "They're just there for everyone to see. Like a slap in the face ..."

Styles revealed that living their lives in front of a camera for the movie didn't take much getting used to.

He explained to the australian.com: "When we started our career, we lived with cameras for 10 weeks on The X-Factor. So we didn't have any problem having the cameras around. It was just a case of getting to know Morgan [Spurlock - director] as we were shooting the movie.

He continued: "And it is exciting because ... through social media or in interviews, you can never really get to know someone. This movie is a great chance for fans and the people who don't really know us to see a different side of us."

One Direction are currently on the US leg of their 'Take Me Home' world tour.

‘This Is Us’ will hit theatres in August.