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Comedian Katy Brand’s witty, tender screenplay for Good Luck to You, Leo Grande has the uncanny feeling of a barely opened-out play. There are four scenes, all set in the same hotel, and essentially just two characters. The relationship between them begins as a performance and gradually turns into something more complex, carnal and idiosyncratically intimate. The double-act here is Emma Thompson’s Nancy, a widow, and sex worker Leo (Daryl McCormack), whom she has hired in order to enjoy a little physical pleasure after 31 years of unsatisfactory sex in her marriage. As she explains between gulps of minibar champagne, she’s not expecting her first orgasm, just a little adventure.

The dialogue is also stagey, revelling in pointed wordplay, such as when Nancy and Leo use different names for the same thing: “pleasure” versus “use”, “anticipation” versus “suspense”. Leo, who is perhaps alarmingly analytical of Nancy from their first meeting, piques the curiosity of this retired schoolteacher when he uses A-level words such as “empirically” and “reductive”; her own years of fearful prudishness are revealed in an anecdote that revolves around the word “concupiscence”.

Is the staginess distractingly bad? Under Sophie Hyde’s direction, Thompson and McCormack imbue their words with enough passion and charm to make the dialogue stimulating – not to mention the growing tenderness between this odd couple as Leo chips away at Nancy’s inhibitions. But there’s nonetheless an unfortunate hint of the debate club about some of their digressions, and as you’d expect, they get stuck into the ethics of legalising the very business they are engaged in, to the vaguest of conclusions.

Still, words can be sexy in this film. Leo is finally successful at initiating sex with Nancy as she talks herself into an erotic reverie remembering a steamy encounter from her younger years. The reverse happens when Leo works himself into a passion talking about the joys of his vocation and there’s no other way to put this: Nancy pounces on his erect penis.

All this frank (and it is, very) talk becomes literal foreplay, as the couple finally tick off the items on Nancy’s sexual bucket list, in a candid montage that is sure to generate some column inches on middle-aged female on-screen nudity. Which seems to be exactly the point, as Nancy’s really looking for a better relationship with her own body. This is a British sex comedy, so inevitably sensual bliss must come after a period of painful reserve. The woman who is at first so undecided that she keeps the price tag on her satin negligée changes into a series of yet more diaphanous blouses and finally writhes naked on her hotel bedsheets. Nancy and Leo’s wardrobes both become more rumpled as the film continues – who has time for ironing when you are chasing an erotic high?

It’s a fairly physical film too: the unmissable contrast between Nancy’s flustered nerves (constantly patting down her smoothly ironed skirt) and Leo’s practised poise prepares us to concentrate on the nuances of posture and touch that define and develop their relationship. Leo flexes pinup poses while Nancy is in the bathroom, and Thompson beautifully plays Nancy becoming overwhelmed by lust as she strokes Leo’s torso. It’s such an unfamiliar feeling, and she is instantly nauseous.

It’d be tricky to imagine this dynamic working so well on screen if the film weren’t so funny. Thompson is a natural clown, gamely launching into extended preparations for giving her first blowjob, and while McCormack is largely the straight man, Brand gives him a few choice lines, including a brisk dismissal of “posh girls” who take up pole dancing as a hobby. Hyde has emphasised Thompson’s input in the finished film, drawing on their shared discussions and the actress’s experiences to create Nancy. And she is by far the most fully realised character of the two – both as a comic figure and a relatable human.

Hyde has a track record of close collaboration with her actors and appears to be drawn to these stories of unconventional, shifting relationships, from the transgender parenting dilemma of 52 Tuesdays (2014) to the marriage-like bond between best friends in Animals (2019). Ultimately, Leo and Nancy are emboldened by keeping their unusual relationship at the level of honest transaction and equally honest hedonism, rather than masquerading as friends or lovers. Truthfulness is of the essence here, no matter how rarefied the vocabulary. As Leo would say, lying about sex doesn’t make any concupi-sense.

► Good Luck to You, Leo Grande is in UK cinemas from tomorrow.