In step with the movie trends

When it comes to showing the funny side of serious situations, you can always rely on Hollywood's Frat Pack - the band of comic brothers led by the likes of Ben Stiller and Will Ferrell.

From Ben Stiller's pathetic stalker in There's Something About Mary, to Steve Carrell's 40-Year-Old Virgin and last year's Knocked Up - a tale of accidental pregnancy written, directed and produced by new golden boy Judd Apatow - the pack, who love to pop up in each other's films, have poked fun at most things.

Now the 'plight' of grown-up children still living at home has been given the comedy treatment in Will Ferrell and John C. Reilly's latest collaboration, Step Brothers.

Empty nesters are becoming a thing of the past in the UK, as more and more children choose to stay with their parents rent-free.

In Step Brothers, produced by Judd Apatow, the two central characters, 40-somethings Dale (Mr Reilly) and Brennan (Mr Ferrell), not only stay at home, but neither of them work and they have managed to remain in a perpetual state of adolescence.

"They never outgrow their adolescent ideas about what's cool, how they'd spend their time when they grew up, what they found entertaining," explains Mr Ferrell, 41.

"It was a lot of fun to explore that, thinking, 'What if you actually became at 40 the guy you thought you'd be when you were 13?'"

Cue juvenile T-shirts, prized drum sets, Chewbacca masks and endless TV dinners for Dale and Brennan as Mr Reilly and Mr Ferrell clearly revel in their roles.

"Dale is an extreme case of arrested development," says John Reilly, 43, with a smile. "His dad's a doctor, so he's never really had to work for anything. He's just into the things that he likes and nothing else interests him at all.

"He likes drums, sling shots, karate and fireworks. He's a guy who already feels like he's got the greatest life ever and he doesn't have to really work."

The film follows single parents Brennan's mum Nancy (Mary Steenburgen) and Dale's doctor dad Robert (Richard Jenkins), who meet, fall in love, marry and then make the life-altering decision to live together.

Sparks fly when the two grown men are forced to share Dale's room and then Brennan makes the huge mistake of touching Dale's drum kit which results in a hilarious childish fight with them rolling around in the front garden.

But the pair eventually bond over a common enemy - Brennan's perfect older brother Derek.

"He's the complete opposite of Brennan," Ferrell explains.

"Derek's successful, handsome, and has everything going for him. He's also tormented Brennan his entire life. Dale comes to Brennan's defence by sucker-punching Derek, and from that moment on, Dale and Brennan are best friends."

When the pressure of looking after two layabouts drives their parents apart, the boys finally resolve to get off the sofa and try and set up their own entertainment company, but will they be able to reunite Nancy and Robert?

Step Brothers came about two years ago after Mr Ferrell and Mr Reilly worked together with writer-director Adam McKay on motor racing flick Talladega Nights: The Battle Of Ricky Bobby. It was such good fun, that as soon as filming wrapped they started thinking about their next project.

"We sat down, had dinner and threw around all these ideas," Mr Ferrell remembers.

"John would play Dale and he would be more of a planner with a business mind - even though he has terrible ideas. He's the one with the drive and initiative, such as it is," he continues.

"Brennan was going to be a little more sensitive, a little more soft-spoken. And he thought of himself as a beautiful singer, but of course he has a terrible fear of singing in public."

Once the trio had come up with the characters, old friends Mr Ferrell and Mr McKay sat down to write the screenplay together.

"I've known Adam for about 12 years," says Mr Ferrell.

"We were both hired at Saturday Night Live at the same time. He had a long career as an improviser and stage performer before he became solely a writer and I think that influenced his directing style. It allows for a lot of freedom."

It was only when they were writing the script, that father-of-two Mr Ferrell realised what a burden the step-brothers would be on their parents.

"When we started writing, we came up with all these crazy scenarios from every kind of brotherly fight we could think of and any adolescent scenario that made us laugh," says Mr Ferrell.

"But as we continued, we really started identifying with the parents."

Fellow parent Mr Reilly, who also has two children, adds: "Imagine if your kids just never really matured and never left the house. I mean, I love my kids, but I really hope they move out eventually!"

Mr Reilly's children were to provide a valuable source of research for him though, as he got into role as Dale.

"I would go home, watching my kids react to not getting something they wanted, or a petty grievance between siblings, and that stuff definitely informed my character. In that way, even though we get into 'adult' situations, the movie has a lot of innocence and joy to it."

Although the characters were sketched out in detail, when it came to filming, the actors soon realised it was going to be a more fluid process than they'd thought.

"There were no easy days on this movie," says Mr Reilly, with a wry smile.

"You might start your day thinking, 'Oh, I just have two lines in this scene, I'm just walking through' and hours later you'd find that your role in the scene had expanded. You always had to come to the set fully prepared."

After a long-running stint on US satire Saturday Night Live, Californian Will Ferrell made his name in the early Noughties for his comedy roles in hits like Anchorman: The Legend Of Ron Burgundy and Blades Of Glory.

But for John Reilly, who hails from Chicago, comedy has only been a recent venture after more serious films from Magnolia and The Hours to Chicago, which gave him an Oscar nomination.

He insists the transition has been a smooth one and that he's enjoying hanging out with the rest of the Frat Pack - for now.

"It's still the same kind of work. It doesn't feel all that different to me; it's just the way the circumstances in the scenes change that make it absurd."