Comedian Ricky Gervais is more of an extra than a professional actor, according to comments made by Hollywood star Ben Stiller, his co-star in Night at the Museum.

In a behind the scenes clip obtained by the Daily Mirror, Stiller explained that the funnyman's acting abilities were frequently impaired because he couldn't stop laughing on set.

Stiller, who luckily saw the funny side of Gervais' infectious humour, joked: "Ricky Gervais has a problem."

"Hes not a professional actor as a professional actor would never crack up," added the US star, who previously played a comedic version of himself in Gervais' hit comedy series, Extras.

Meanwhile, former Grease star John Travolta has become the latest actor to sign up to star alongside Gervais in Extras after sharing a chat show sofa and some flashy dance moves with the comic.

Travolta has reportedly agreed to star in one of the last episodes of the series after appearing on Jonathan Ross' BBC One talk show last week, when he flaunted some of his Saturday Night Fever moves, prompting Gervais to re-enact his own infamous David Brent-style dance from The Office.

05/04/2007 07:59:53