That first flush of romance

"There was a lot of very technical jiggery pokery that had to go on to make sure everybody was happy," says Ritchie. "But it was fun, the whole shoot was fun."

The two can laugh about it now: that's what time's passing and having careers on the up-and-up can do for an actor's disposition. Besides the new movie, Ritchie has just been in White Heat, the all-our-yesterday's BBC TV drama, and Karan is soon to appear in the Simon Pegg comedy, A Fantastic Fear of Everything.

Directed by Nigel Cole, All in Good Time started out as a 1960s play by Bill Naughton. The film version is by Ayub Khan-Din, the writer of East is East and West is West. Like those box office hits, it is a blend of bittersweet comedy and sharply drawn drama. Ritchie was drawn to it because it doesn't take an easy path between the two.

"I don't think it is a comedy, but it is very funny. Certainly from my character's perspective he doesn't find it funny at all. But on the flip side it does have you belly laughing as well."

Karan, 29, saw East is East when she was at school and thought it was brilliant. Though it had particular things to say to her as someone whose parents were from Sri Lanka, she thinks it spoke to everyone. "It's just a very British film as well – there was a universality about it that made it really sing."

Karan is a Londoner, Ritchie, 25, is from Suffolk. Neither comes from an acting family. His parents run their own business, and her father is a financier. For a while she followed dad's career choice, working in the City in mergers and acquisitions. It was not a merger that lasted, however. "I quickly realised that life is too short to be spending it in an office in front of a computer all day."

She had acted through school and university (Oxford, PPE), travelling to the Fringe in 1999. Acting had been what she always wanted to do, but the move was hard to sell to her parents and herself. Her parents were "desperately" worried when she gave up the City job. "They were like, are you crazy? I was worried as well. But you have to do what you love, what you feel compelled to do."

Ritchie's parents fretted less, on the surface at least. After gaining straight As in his A levels, they thought he would go to university, but he opted for drama school, which he then left after getting acting work. "There's no greater teacher than to be thrust into the belly of it all. I was just eager to get out there and start learning on the job."

Neither has played it strictly by the book, but it worked out OK. When your first film job is a Wes Craven comedy, The Darjeeling Limited (Karan), and a Roland Emmerich blockbuster, 10,000 BC (Ritchie), "OK" is probably understating it. Very different films, but the experience was the same for both young actors.

"I didn't know what the hell I was doing on set, didn't know what the protocol was, anything," says Karan. Adding to the difficulty was shooting on a train winding its way through India.

"Pretty nuts" is how Ritchie describes his first blockbuster experience. Up to that point, acting had meant low budget and high culture. "One week I was in rainy Essex in a Portakabin reciting Shakespeare, the next I'm on a mountain in New Zealand dressed as a mammoth hunter."

The experience prepared him for Prince of Persia, then The Lovely Bones, Peter Jackson's (coolly received) adaptation of the Alice Sebold novel. Appearing alongside the likes of Susan Sarandon and Mark Wahlberg, Ritchie would have been more nervous if not for Jackson's on-set style. "He's just the most relaxed guy ever. Three-quarter length shorts, old shirt, bumbles around with his cup of tea."

After Darjeeling, Karan was next seen in St Trinian's, which brings us to an awkward part of the interview. It's not All in Good Time, bedroom scene awkward, but a fair bit of um-ing and ah-ing and nervous laughter goes on, prompted by my wondering if Karan had any doubts about appearing in the Russell Brand/Rupert Everett naughty schoolgirls comedy.

"What did you do?" asks Ritchie. "I haven't seen it. Do you get a bit frisky in it?"

Karan laughs. "No, we just looked pretty awesome in our school uniforms. We had our suspenders and our heels and our little skirts."

She was delighted to get the part at the time. "I'd just started, it was my first year out of drama school and I was so happy to work and be paid to work." She did not sign up for the sequel.

The first few years out of drama school are all about finding your feet, says Ritchie. All in Good Time is special to them both, he says, because they have a chance to develop a character in detail, to really show what they can do.

Playing Ritchie's mum in the film is Meera Syal. The writer-actor once told The Herald of the stereotyping she encountered at the start of her career, saying: "Being young and Asian meant that generally people would be looking for the girl running away from an arranged marriage."

Both admire Syal's style and attitude. "She's had to earn her stripes," says Ritchie. "She's written roles for herself, she's had to graft and carve out her career."

Is it different today, for Ritchie and Karan's generation? "Whether you like it or not, this business, probably any business, will follow the grooves that have been set before," says Ritchie. So if you're eight foot tall, he says, you are probably not going to be cast as Romeo.

"Every actor has to be aware you are a product to some degree and you have to be aware of what your casting is. It doesn't mean you have to obey that." He has never found it a limitation. "If you work hard at what you do you can pretty much do anything."

Karan, similarly, has never felt there wasn't a part she couldn't go for. "I really feel it genuinely comes down to who is right for the part."

That said, Ritchie does not discount that there is a problem.

"I've got friends who have changed their names recently and all of a sudden they are getting seen for things. So there is a prejudice in the business, but I don't see anything to be achieved by sitting around moaning about it. You've just got to get on with things in any job you do."

All in Good Time opens on May 11.