Colin Firth and Emma Stone on starring in Magic in the Moonlight: Woody Allen's 1920s film

They might not be the oddest couple ever to enter a Woody Allen film, but Colin Firth and Emma Stone are still an unusual pairing. He's the British, 54-year-old, Oscar-winning actor of The King's Speech, a national treasure-in-waiting after a 30-year career that began with 1984's Another Country.

Today, sitting next to each other in a Paris hotel, they're talking up Allen's latest, the lightweight, 1920s-set romantic fantasy Magic in the Moonlight, in the absence of the writer-director, who – regular as clockwork – has just finished shooting his next movie (his 47th) in Providence, Rhode Island.

Both newcomers to Allen's world, it's something of a dream come true, especially for Stone. Her mother showed her Annie Hall when she was young (and their family dog is even named Alvy, after Alvy Singer, Allen's character in the film). She's also a huge Diane Keaton fan, as is Allen by all accounts. "I think Woody would like to be Diane Keaton in a Woody Allen movie. He is the biggest fan of Diane Keaton and he knows how I feel about her. He loved to regale me with how wonderful Diane Keaton is, and how much she made him laugh."

Stone may not be the next Keaton, but she's certainly Allen's current muse – and will appear in his next film, with Joaquin Phoenix. "Well, [I'm not sure] if two counts as a genuine Woody heroine!" she shrugs, but she's clearly delighted by the idea. "What I've always wanted to do are movies like movies that Woody makes… movies that are heartbreaking but have a lot of levity. That's the way I see life. I don't see life in these split ways."

As for Firth, snagging a lead in a Woody Allen movie is an achievement, given he didn't even get cast in a comedy until 1997's Fever Pitch, after a decade-and-a-half of straight roles. "I wasn't really asked to be [funny]," he admits. "I thought that door was closed to me. I think if you've been at it for 15 years, and no one has ever asked you to say a funny line, then it's fair to assume that you're going to be one of [those] actors that only ever do one or the other."

Suddenly, after Fever Pitch, Firth was deemed funny by the industry – pitching up in the Bridget Jones films, Love Actually and even the St Trinian's franchise (where he mocked his own infamous "soggy shirt" scene as Mr Darcy in the BBC's 1995 Pride and Prejudice adaptation). When it comes to making audiences laugh, he's all too aware of the pitfalls. "The trouble with comedy is that it doesn't have anything to fall back on. If somebody doesn't like it, it looks cheap… when it doesn't work, it's spectacular. It's like a soufflé collapsing."

Certainly, that's a risk with Magic in the Moonlight: with its gossamer-thin plot, it's more an amuse-bouche than the hearty three-course dinner that Allen's last film, Blue Jasmine, served up. Firth plays Stanley Crawford, a magician who performs as a Chinese conjurer, Wei Ling Soo. Invited to the Côte d'Azur by an old friend, he's introduced to a wealthy American family who have been taken in by a young lady, Sophie Baker (Stone), who purports to be a mystic able to commune with the dead.

While Stanley sets out to debunk Sophie and expose her as a gold-digging fraud, it doesn't take a fortune teller to realise that love will blossom between the two. Of course, Allen has never been afraid to pitch his male characters with much younger female accomplices – right back to 1979's Manhattan, when he played a middle-aged divorcee dating a 17-year-old Mariel Hemingway. More recently, we've seen Larry David and Evan Rachel Wood get together in Whatever Works (despite the 40-year age gap), while Allen himself is married to Soon Yi-Previn, 37 years his junior.

In this case, the age difference is "not mentioned in the script, it's not written with that as a feature", says Firth. Nevertheless, he was concerned. "I was very aware of it at the beginning, when I found out who is playing the roles. At the time I thought, 'I'm going to seem so old and washed-up next to Emma Stone!' I wasn't exactly relishing being on the unflattering end of that. Once we were under way, other things came to the fore. But people are going to draw attention to it. I get it. I really do understand. It's noticeable."

As he notes, in the US reviews have drawn attention more to the age gap than in Europe. "It's probably about their attitude [in America] to what is conventionally represented." Stone interjects: "It is understandable, because there is a huge 'trip' in Hollywood," she says. What does she mean? "Women over a certain age have a much more difficult time being cast, no matter how old they are." It seems she's suggesting that if there's an outcry, it's not for reasons of impropriety but rather for those of inequality in the workplace.

While there is some truth to that, it hasn't gone unnoticed that the plot bears resemblance to Allen's recent personal troubles. As Film Comment writer Nicolas Rapold put it, "the film's suspense derives primarily from the spectacle of an older entertainer trying to prove that a young woman is lying" – an unfortunate coincidence, given the revival of accusations last year by his adopted daughter, Dylan Farrow, that he abused her when she was seven, something Allen vehemently denied in a letter to The New York Times in February.

According to its stars, Magic in the Moonlight has other concerns. "I think in every movie he's wrestling with duality; he's in this tug-of-war," says Stone. And in this one? "I would say love versus rationality." Certainly that fits with the cynical Stanley, a man of logic who gradually comes to believe in Sophie's gift of being able to communicate with the spirit world; the way both Stone and Firth see it, their on-screen relationship mirrors that of Henry Higgins and the flower-girl Eliza Doolittle whom he takes it upon himself to "transform" in Pygmalion.

In the end, Magic in the Moonlight tries to understand the essence of love – "an explicable, unseen force that seems to have a grip on us all," says Firth. Whether audiences will buy into this fantasy, the 78-year-old Allen isn't about to change now. Word has it that in his next film Joaquin Phoenix plays a professor who falls for one of his students – who may or may not be played by Stone. "That's top secret," she says. Then again, given Allen's reputation for carving out inter-generational affairs, it'd hardly be a surprise.

'Magic in the Moonlight' opens on Friday