Radical switch sees Reitman put a fresh focus on detail

"Working with sweat is similar to working with snow. There are 10 different ways to do snow and you use them for different reasons. There's the kind of snow you put in background, in foreground, snow that falls, snow that is made out of soap, or foam. Sweat is very similar. Fake sweat can be made out of a variety of things. Is the sweat going to drip or is it going to be a sheen, on the forehead or their arms, is it going to stain?"

The perspiration came into play because Labor Day takes place during a long, hot summer in New Hampshire. Kate Winslet and Gattlin Griffith star as the mother and teenage son out on a shopping trip who run into a man desperate for a lift out of town. What follows is seen through the eyes of the teenager as the temperature rises inside and outside the family home.

The book is based on the novel by Joyce Maynard, and as with many of the writers he has worked with, Reitman sought her advice. "They bring up things that I miss or something in their book I've glossed past and didn't realise how important it was. Even little details."

Labor Day will be seen as a radical departure for the director of the hard-hitting Young Adult and the satire Thank You For Smoking. He had not set out to change his filmmaking style; it was a case of needs must. "I just fell in love with the book and then I had to learn how to make this kind of film."

High on the list of things Reitman did not have to worry about were Winslet and Brolin. Winslet, he knew, would bring strength and vulnerability. "With Josh it's masculinity in an era of boyishness."

Brolin has the air of a latter day Gary Cooper, I suggest. "Yeah, but even fiercer," Reitman laughs. "He and Clooney and Aaron Eckhart, I've worked with a few now, men in the era of boys. But also Brolin has immediate chemistry with, seemingly, everybody. He falls in love with people and people fall in love with him. I knew in the casting these two would be great together."

Reitman has always displayed a sure eye for casting and story, from Thank You For Smoking, with Eckhart, to Juno, with Ellen Page and Michael Cera. Perhaps his real skill lies, however, in putting familiar actors into unfamiliar settings, as with George Clooney playing a working schlub whose unenviable job it is to fly round the country making people redundant. How do you make Clooney look ordinary?

"I wish I could say I used my directorial prowess to mine that from George Clooney. He works so effortlessly. If he connects and he knows what the movie is about, and he certainly did in Up in the Air, he does the smallest, most beautiful little moves that bring scenes to life. He can do it with a glance, with a turn of his head."

Then there was Charlize Theron, who ripped the bones out of the part of an acerbic writer in Young Adult. Like Juno MacGuff, Mavis in Young Adult was not an obviously audience friendly female character.

"I don't think any studio thought this is the next Harry Potter," says Reitman when asked if it was difficult to get the green light for Young Adult. "I was coming off the success of Juno and Up in the Air, and it was Charlize, and it was kind of cheap and they went for it. I don't think the content thrilled them but it sure thrilled me. I just thought I know exactly who this woman is and I've never seen her on screen."

Reitman, 36, is the son of Ivan Reitman, producer and director of Ghostbusters and Kindergarten Cop. His childhood was spent at his dad's office, watching films spring into being. Though it was a natural move for the son to follow the father into films, it was one Reitman Jr resisted initially.

"As a teenager I was aware of the opinions on nepotism and the children of famous filmmakers trying to be directors themselves and failing miserably. I didn't want that life. I went pre-med at school. For a moment I thought I'd be a doctor."

His father visited him at school and convinced him otherwise. "He gave me the confidence to at least give it a shot." Reitman Jr started out quietly and small, making short films and submitting them to festivals, reasoning that he could build a name for himself that way. Dad remains to this day a source of advice. "I don't even know where to begin on the amount of advice he has given me as a filmmaker."

Where Reitman needs no help is in making films that ignite debate, with both Juno and Thank You for Smoking generating controversy. The good thing, he says, was that both sides thought the movies were theirs. One week, for example, he received invitations to speak at both an anti-smoking conference and a libertarian convention.

He explains: "Christopher Buckley, the author of Thank You for Smoking, once told me a great book and a great movie, when they really work, should be a mirror. You should just look at the screen and see yourself. I thought that was perfect. I never want the audience to know how I feel about the subject, I never want them to think I'm telling them this is how you are supposed to feel."

Reitman's next movie will be Men, Women & Children, a comedy starring Emma Thompson and Adam Sandler (now there's an example of left field casting). It's about how the internet has changed the love lives of teenagers and their parents. As with Up the Air, Reitman seems to have a knack of finding the right subject at the right time.

"Complete luck," he says. "I started writing Up in the Air seven years before I made it. When I started writing it, it was the tech boom, unemployment was at an all-time low."

We return to Labor Day. Besides a whole lot of sweating going on ("I remember getting notes from week one from the studio, going 'Why are they so sweaty?"), there was baking, too, with Brolin's character a dab hand at making peach pies.

"Josh was making a pie every single day. You'd visit him at his cottage and he'd be there in an apron. Out would walk this picture of masculinity but he's talking about crusts."

Reitman smiles at the memory.

"We all ate a lot of pie."

Labor Day opens in cinemas tomorrow