The Weekend's Viewing: Me and Mrs Jones, Fri, BBC1 Friday Night Dinner, Fri, Channel 4

It's an odd thing, comic exaggeration – absolutely essential to comedy but also potentially lethal if you get the dose wrong.

Take Me and Mrs Jones and Friday Night Dinner, for example, both comedies of domestic life, both of them making an appeal to a sense of shared experience. And one of them works and one of them absolutely doesn't. The one that doesn't is Me and Mrs Jones, which is odd really. It's written by Oriane Messina and Fay Rusling, who have Green Wing and Smack the Pony on their CV, and its comic premise is perfectly workable – a single mother who finds herself falling for her oldest son's best friend. You can play this cross-generational attraction for anguished drama – as ITV's Leaving did recently – but its embarrassments obviously have comic potential too. So why doesn't it bite?

My own explanation would centre on something Gemma's daughter says to her as she drops her off in the playground, after a flustered school run full of slightly effortful blunders: "Stop being a geeky loser." You've hit the nail on the head there, kid, I thought. That, or something like it, is what's written in Gemma's character notes, and it's why we've already had to endure one of those unconvincing scenes in which someone stammers and over-protests after being misheard. Is she really this dim, you think, or is she just written this way? The question doesn't go away as Gemma is forced through a number of over-familiar comic set-pieces – the clumsy answerphone message, agonising over what to wear for a date – all the time behaving not as if she's directed by a recognisable inner psychology but by the need to appear as ditzy as possible. At times, it's desperate, as when Gemma appears from a changing room having tried a dress on over what she's already wearing. Sure. That happens a lot.

Even more problematic is her inconsistency. Gemma is flustered when she really doesn't need to be, but unperturbed when awkwardness might actually make some kind of sense. "Uh! I feel like a teenager on her very first date," she confides, as she gets ready for a night out in front of her son's handsome young friend. A couple of lines later she's blithely explaining to him how she'll use her unshaved legs as contraception. So she's reduced to gibbering silliness by a man she doesn't appear attracted to and coolly overshares with one who notionally has got under her skin. I've never been a single mother in such circumstances, it's true, but I'm still not convinced that's how the world works. The casting doesn't help either. Sarah Alexander looks far too young to be convincing as the mother of a grown-up son and isn't the kind of comedy actress who can finesse the thing into caricature. But the real problem is a script that repeatedly requires her to behave with wild improbability. "I may have slightly over-reacted," she says at one point. Just a bit, Gemma, just a bit.

Robert Popper's Friday Night Dinner also contains moments that are tricky to defend as psychological realism. "He looked like Hitler," said odd neighbour Jim, after meeting the grandmother's monstrous new boyfriend. "It's not Hitler, is it?" But Popper's comedy has an internal consistency that makes it work. Jim is strange enough to say something like that. You feel absolutely confident that none of the other characters would, because they stay true to type. The monstrous boyfriend works too, even though he's a kind of cartoon of belligerent old age, because the forms of his unpleasantness are simultaneously unpredictable and credible. "One rule I have when I'm in a vehicle," he barked when driving the boys out on an errand. "Complete silence! Not a word!" After which, he slowly drove into the wall in front of him. Like a lot of good comedy it's simultaneously over the top and understated.

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