Four years ago, Ben Lewin was surfing online for disabled sex. The survivor of boyhood polio who had worked as, variously, a jeweller, a barrister and a movie director, Lewin found himself hitting 60 and with his career on the skids. His agent had a suggestion: write a sitcom, vaguely based on his own life, about a man who trades the use of his disabled parking badge for erotic favours.

Lewin agreed it had potential. "I'd just look in the mirror and say: 'Well, there's a funny idea'". He began the research, and devised a working title (Gimp). Was it really autobiographical? Lewin looks coy, in an Aussie fashion. "Well, ask my wife if she has enjoyed the benefits of privileged parking. I'm sure she doesn't complain." He sighs agreeably. Do we all trade things for sex? "Oh yes. For me, it depended on what stage I was at and how desperate I was feeling."

But what Lewin stumbled across put Gimp permanently on the backburner. It was an article by Mark O'Brien, a poet who lived in Berkeley in the 1970s, forced to spend most of his days in an iron lung – again, as the result of polio. Though prone to falling in love with his carers, at 38, O'Brien found himself still a virgin – so he employed the services of a professional sex surrogate (more medical worker than prostitute). Reading his piece, says Lewin, was "a burning-bush moment".

"I had the self-awareness to know I'm very hard to reach emotionally and that if something could do that as effectively, there was a powerful story inside it. I was worried at first that I had too special a point of view. But then I was persuaded that it was a universal kind of story, and I used my specialness to persuade people that I was the right guy for the job."

He was. Fox Searchlight splashed out $6m for distribution rights following The Sessions's premiere at Sundance last year. Next Sunday it's up for two Golden Globes, for John Hawkes (as O'Brien) and Helen Hunt (as the surrogate).

Yet despite this acclaim for its cast, what distinguishes the film is its tone: the light touch and sly wrongfooting. For all the frank chat and full-frontal nudity, this is a romance, the chirpiest weepy you'll see. Like his film's hero, Lewin is a charmer, sweet and circumspect, who speaks of his disability in terms of comedy, not struggle.

He feels the deepest kinship with O'Brien, who died in 1999, over their ability to connect with the opposite sex. "Mark had a special knack of being able to tell a woman what she wanted, to use the power of language and imagination - which resonates more for women, I think. While I don't want to underplay what a shitty hand of cards Mark got dealt, he probably still had some really high-quality ladies in his life. And there are a lot of able-bodied guys who go through life without experiencing that."

Lewin is an unabashed natterer, socially polished behind that larky exterior. I first met him in Toronto, at the TV junket for The Sessions, in the hospitality area where journalists scribble out questions and stock up on sandwiches. Lewin, wife and daughter in tow, sat down at my table for their own lunch and started talking, without reference to the movie nor his part in it.

"I have always enjoyed the company of women," he says in London a month later. "I feel that there is a way of reaching their sensibilities other than through brute force." Part of the knack is knowing your target. "I see givers and takers. Some women have a much more nurturing quality. And being a taker myself I would gravitate towards that. There are men who are the opposite, who are really more attracted to bad girls. The ability to recognise what pushes certain people's buttons is crucial."

And it is studying others that appears to tickle Lewin. In a sense, then, the necessity of talking about his physical affinity with The Sessions's subject matter must be frustrating. Here is a man whose mantra had always been to write about that which you don't know. He plays down the extent to which his condition affected his childhood ("I am more connected to the world of the imagination, but you don't have to have polio to do that"), expresses disgust at what he sees as a mushrooming trend for egocentrism.

"I remember when our oldest daughter was going to college we were looking at these sample essays that people write for their applications. They were all very self-centred. I thought: this is wrong. Why don't they treat writing the essay as a learning experience rather than a navel-gazing experience?" He looks unusually upset. "Particularly with middle-class people, self-absorption is a struggle."

Lewin's own history is one of the benevolent eavesdrop. Born in Poland in 1946, to parents whom he says stayed together unhappily for the sake of their children, the family decamped to Australia when he was a toddler. There, they opened a corner shop. "It was almost like the Rovers Return. Everyone came and spilled their guts and vented about their neighbours. I still felt we were aliens in that community, but we were accepted. In retrospect it did stimulate me creatively."

After ditching a legal career, Lewin moved to London in the late 70s, where he was all set to do an adult education class in gilding. But the man next to him in the queue cautioned against it. "He said: 'The teacher's a real bastard. Do continental patisserie.' I changed lines and it was one of the most pleasant years I ever spent. I so looked forward to my classes. I was the only guy, apart from the teacher, who was the pastry chef at Fortnum and Mason's."

His fellow students were all middle-aged women. "They would talk about their husbands in this conspiratorial, criminal way. I thought the following day I was going to read about some grisly murder in the paper. They would enrol year after year just to get away from their dreadful marriages. That was one of the fascinating parts of it, learning – my God, is this what happens to marriage after so long?"

Lewin moved to Hollywood in the 80s, and was sure to always take cake to pitch meetings. "If I called a couple of weeks later I'd mention the food and they'd say: 'Oh, it was delicious! Who's your agent again?'"

What was his speciality? "I'm very good at florentines; really top class Florentines. And lemon tarts in a very traditional French style, very complex apple cakes. I tend not to go for the multilayered cream disasters. Too fatty. You feel awful afterwards. I like things where you just have a little taste and you've got it. You don't need to gorge."

The Sessions follows a similar recipe: no fat, little treacle, weighing in at a slimline 88 minutes. And although Lewin's previous movie career isn't half as chequered as legend might like to suggest (a Judy Davis thriller, a Jeff Goldblum farce, an Anthony LaPaglia romcom, an episode of Ally McBeal), he agrees that had he shot it 30 years ago, he'd have been likely to overegg the pudding.

"I might be tempted to do sex scenes in a more conventional way, as if they were the pages of Playboy coming alive. When I was younger I would just stress myself out like crazy. These days I take my crossword to the set with me and I'll nap every now and then and get involved in trivial conversation. The result is so unpredictable you've got to learn to love the process. That might be the only thing you get to enjoy."

And there's the rub. For all Lewin's happiness at this late career revival, – "I feel in some ways that we've won the lottery after buying tickets for 40 years" – you sense the shadows snapping at his ankles. Being the oldest Sundance kid in town has its drawbacks. Perspective isn't always a blessing. "I'm used to having really nice stuff spaced out. I'm a little bit worried about the idea that this time next year it will all be over. Nobody will want to rush up and talk to me any more."