Readers of Clare Allan's terrific novel Poppy Shakespeare will have been struck immediately by two things in last night's Channel 4 adaptation. The first was that Poppy was played by a black actress, Naomie Harris. There is, it is true, no mention of Poppy's colour in the book but the narrator, N, is such a cheeky observer, keen on epithets such as “Middle Class Michael”, you would have thought her ethnicity might have got a mention. The casting, happily, proved inspired. Harris's beauty stood out all the more among the drab community of NHS psychiatric outpatients - known by N as “dribblers” - confined to North London's Dorothy Fish Ward.

The second thing you realise is that the director Benjamin Ross has made the story less funny and satirical. The tragi-comedy is now more tragic. The relief the reader feels when N finally escapes institutionalisation is for the viewer non-existent. Instead, the narrative focuses on Poppy's descent from successful mother and career woman into a vegetable planted on the hospital's highest storey. Poppy originally becomes entangled in the NHS web after filling in a psychometric assessment form provided by a government employment initiative. Told that unless she enrols at the Fish as an outpatient she will be sectioned, she enters determined to prove herself sane. But to claim legal help to prove this, she must first make herself eligible for “mad money” by declaring herself insane. This is Catch-22 in reverse.

Most people at the Fish have a different problem: a morbid fear of being released. Yet being released early - often with suicidal results - is exactly what is happening as the hospital strives to meet targets by “curing” its charges. Michael, a tank-top-and-tie pressure group veteran, the book's Middle Class Michael, is on to the Government's little game but, because he is “mad”, no one takes any notice. The only person who is improving under the Fish's regime is N, who bonds with Poppy after being told to show the new girl the ropes. This she does only too well. In learning how to fake mental instability, Poppy makes herself ill even as N becomes a more and more competent teacher. In this story, as in life, you are the roles you are required to play. By the end, N has assumed Poppy's poise, her dress-sense and free spirit.

Anna Maxwell Martin's performance as N was remarkable. In a pudding bowl haircut and red puffer jacket, she looks not so much childlike as babyish. Her face is at first glance blank; in fact, however much she tries, it is unable to conceal her emotions. For N, the friendship with Poppy is a romance, her first real relationship since her mother killed herself when N was 4. The book, however, lets us see how N kills the things she loves by stealing Poppy's personality. Here, in Sarah Williams's adaptation, I could not see how N behaved in any way culpably towards her friend, who is a victim, instead, of an unlikely NHS conspiracy. It was a touching, lovingly acted film, with a highly evocative score by Molly Nyman and Harry Escott, but it replaced Clare Allan's angry, satirical brio with a futile melancholy. Now read the book.

Brian Keenan, it has often been observed, had no luck. After years of captivity in Lebanon, first in solitary confinement and then with John McCarthy, he was desperate for company. And when it came, who was he landed with but the pompous, sanctimonious Terry Waite? Because Keenan appeals to our love of the garrulous Irish and conforms to the Christian model of wisdom through suffering, no one stops to ask what Waite felt about Keenan. On the basis of Brian Keenan: Back to Beirut (BBC Two), in which the former teacher returned to the Lebanon 17 years after his release, I would not have endured more than a day of his spiritual and literary insights, all of them unencumbered by local political knowledge.

Keenan compared Beirut to a beast that ate itself, speculated on what the eyes of dead Palestinian refugees were “saying” on posters (“Why me, why, why?”), got charmed by a posse of “innocent” little girls who wanted him to be their teacher (we assume he declined) and stood in a river pleading with the god Adonis for “a few more years”. “I'm not trying to be poetic, but I get a sense that the winds in the mountains blew things off me,” he concluded. I hate to think what poetry he might come up with were he to try.

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