By Michael Osborn
BBC News entertainment reporter
Ant McPartlin (right) plays straight man to Declan Donnelly
Ant McPartlin and Declan Donnelly are breaking out from television for their first major role on the big screen.
The duo, well-known for hosting shows such as Saturday Night Takeaway and I'm A Celebrity... Get Me Out of Here, play two men who recreate footage of a post- mortem carried out on an alien and convince the world it is genuine.
Alien Autopsy is based on the true story of Ray Santilli and Gary Shoefield, who claimed their 1995 film shed new light on the Roswell UFO crash site, sparking a storm of interest.
"We couldn't believe this great story. When we read the script we thought it was the one for us," says Donnelly.
"We'd been offered films in the past, but none of them came up to the mark."
The pair had to screen test for the roles in TV director Jonny Campbell's debut feature film.
Having to go through the audition process made the TV stars warm to their first major film project.
"It made us think it wasn't just an Ant and Dec vehicle - they were going to make it whether it was with us or not," says Donnelly.
McPartlin and Donnelly have been cast against their usual small screen roles in the film, with Ant transformed from a wisecracking buffoon into the serious, bespectacled Gary, while Dec is the rougher-edged, louder Ray.
The 'alien' is a modified dummy with raw meat for innards
The pair had to be convinced that such a role reversal would prove a success.
"We instinctively thought we would play the opposite parts, but the writer and director said they would prefer it the other way round. Now I've seen the finished product, I couldn't imagine it the other way", says McPartlin.
The film, containing gore, bad language and some dark undertones, is a stylistic break for the pair and may require their fanbase to make some adjustments.
"I just hope they go and see it," says McPartlin. "I hope it's well-received."
"It's not too dissimilar tone-wise to what we usually do on TV. But there's also a slightly serious edge to it."
Ant and Dec's TV break came with roles in BBC children's drama Byker Grove. They enjoyed a brief musical career before presenting became their stock-in-trade - making a return to acting a shift in gear.
"It took a while to change from our Saturday night presenting head back to our acting head, but it was all down to Jonny Campbell.
"We found the more real we could make the performance, the funnier it would be," says McPartlin.
Comic Jimmy Carr is among the film's supporting cast
But the pair are not about to turn their backs on their extensive work on the small screen in favour of a film career.
"It's certainly a one-off at the moment. We have a lot of television commitments," says Donnelly, with the pair signed to ITV until the end of 2007.
"We want to see how people react to us doing it. We're certainly not going to launch an assault on Tinseltown. This might be the only film we ever make, so we're going to enjoy it."
The pair add that they are looking forward to promoting the film overseas, where they will be free of their UK celebrity.
"It'll be interesting for us to be seen as just two actors. We're not taking it for granted that we'll be known," says McPartlin.
Alien Autopsy is released in the UK on Friday.