The Freddie Fox Club

The Fox acting dynasty grows ever larger. Just when you thought there could be no more famous Foxes until baby Winston, son of Laurence Fox and Billie Piper, comes of age, along comes another chip off that magnificent old block: Freddie Fox. He has the patrician vowels of his father, the actor Edward, the sweetness of his mother, the actress Joanna David, and an eccentric charm that's all his own. He arrives for our interview wearing a leather newsboy cap, a woolly cardigan, a cravat, a homemade necklace of safety pins and paper clips, and a huge eager smile.

Frederick Samson Fox (after the red-bearded Victorian industrialist Samson Fox who made the family's fortune in corrugated iron) is the fourth generation in a theatrical dynasty that started with the playwright Frederick Lonsdale, his great-grandfather on the maternal side. Angela Worthington, his grandmother, was the precocious girl Noël Coward described in the music-hall song 'Don't Put Your Daughter on the Stage, Mrs Worthington', who married Robin Fox, the theatrical agent. Their sons are Edward, star of The Day of the Jackal and Edward & Mrs Simpson; James, star of The Servant and Performance; and Robert, producer of plays including Another Country and films Iris and The Hours.

Born 15 years after his sister, the actress Emilia Fox, Freddie was basically an only child. 'Spoilt rotten I think,' he says. His earliest memory is sitting on the shoulders of his godmother, Penelope Mortimer, picking blackberries. From his godfather, John Mortimer, he learned a lot about comic timing: 'He was an incredible raconteur.' The two grew close and Freddie acted in Mortimer's translation of a Feydeau farce at school. For a while he wanted to follow his godfather to the bar as a barrister, but boarding at Bryanston he started acting and directing. One evening he was sitting in the common room with friends when The Day of the Jackal came on the television. 'I couldn't believe this cruel, cruel steely-eyed assassin was my dad. He's a complete hero to me. I love watching his films and learning from them, though obviously I hope I'll do different sorts of things from him.'

Such as playing a New Romantic transvestite pop star. In the forthcoming BBC Two biopic Worried About the Boy, starring Marc Warren as Steve Strange and Douglas Booth as Boy George, Freddie plays Boy George's friend and fellow gender-bender, Marilyn. Stills from the production show Freddie looking luscious in full drag. He relished the transformation, but his girlfriend, an American fashion student, found it harder. 'When I knew I was going to audition for the part, we sat down in front of my laptop and looked at all the footage of Marilyn, and she went very quiet and then said: "Is this really what you want to do with your life?"' Later, 'she got quite into the idea that she could have a boyfriend and a best friend and someone to be competitive with, all in one person' and helped him dress up for the final audition: 'pink shoes, red denim trousers, a T-shirt so low-cut it showed my nipples, lots of chains and my hair in rubber bands all over my head, just to show I could do it.' The relationship did not work out – she went back to America – but he landed the part, and spent his time on set wandering around in full make-up and a wig listening to old Marilyn interviews he'd recorded to help him get the 'covered, glottal-stopped East End accent'.

He has the perfect complexion for the part, and looks like he barely needs to shave. 'I'm certainly not a stubbly beast,' he says. 'If I don't shave I look like a peach with fuzz on it – not a good look.' It's a peachy family: Dirk Bogarde once described James Fox, with whom he starred in The Servant, as having 'a muted quality of corruptibility. This man could spoil like peaches.' Freddie, however, is a blither spirit, full of whimsy. A first-time voter, he studied all the party manifestos but the deciding factor for him was the candidates' apparel. 'No one with a bad tie is getting my vote. Some Lib Dems wear the most shockingly awful ties.' He played the cello for 15 years and loves watching Barenboim concerts. His favourite place on a Sunday is Columbia Road flower market (he is soon to move nearby) where he buys irises. 'I love their slash of vibrant purple. Always my favourite choice for sisters, mothers, girlfriends'

Learning his lines, Freddie goes on his own to Joe Allen, the luvvies' favourite, settles down with 'a salad and a spritzer' and chats away to himself. 'I must look like an exhibit from the zoo. Often people give me very funny looks on the Tube.' His latest line-learning has been for Any Human Heart, the forthcoming Channel 4 adaptation of William Boyd's novel, in which he plays a younger version of Sam West's character. 'I'm having moles put on to look like him and he's having contact lenses to look like me.' Before filming, he already knew William Boyd, having met him at 'one of those swanky parents' garden parties where you go as a kid and stand in the corner and munch smoked salmon sandwiches'.

He makes light of his privilege, doing a fruity, high-pitched imitation of him and his father arguing about Shakespeare. 'No, Dad, the line is this' 'Well, it isn't in the first folio.' 'Well, it is in the second quarto.' His father always wins, but one day Freddie may catch up. He's highly rated by his drama teachers at Guildhall, having, according to Christian Burgess, 'an intuitive sense for performing that's very rare'. And, he adds, 'He is just so funny.' The director Ed Dick, who cast him as Hamlet, says, 'His acting is alive, somewhat anarchic and infused with an infectious sense of fun. He reminds me a lot of his cousin Laurence [star of the Inspector Morse spin-off Lewis], but he's his own person. He's charismatic, talented and very watchable.'

'My cousins Lydia and Jack and Laurence are all incredibly naughty and charming,' says Freddie. Family Christmases sound high-spirited, with Freddie 'opening other people's presents and knocking over the Christmas tree and pretending it was Laurence' They are not a religious affair, says Freddie, 'but my uncle James did turn to religion very seriously [he took ten years out to work as an evangelical Christian] and it did wonders for him. I haven't done that, but I would never say I wouldn't. Although at the moment I don't need to.'

He has to rush off, inexplicably adopting a Northern accent as he explains he's going to 'a Gucci party – very exciting!' And, cheerily wishing me 'lots of love', the latest Fox disappears.

Worried About the Boy is on BBC Two on Sunday at 9pm