Terry Pratchett: Choosing to Die, BBC2, Monday<br/>Castle, Channel 5, Friday

Terry stares death in the face &ndash; and opens all our eyes

The backlash was inevitable: 898 complaints to the BBC overnight about its decision to show a man taking his own life on Terry Pratchett: Choosing to Die.

Four senior peers, too, came out against the documentary, calling it "repugnant" and "disgraceful", while charities and religious groups have accused the programme-makers of providing propaganda for euthanasia. Do they have a point?

From the outset, it was made evident that Pratchett believes that putting oneself through the process of assisted death is a personal determination – that freedom of choice is paramount in this area. At the documentary's crux was the novelist's own dilemma: diagnosed with Alzheimer's three years ago, he now struggles with short-term memory and, as he said, "I know the time will come when words will fail me. When I can no longer write books, I'm not sure that I will want to go on living."

He thus decided to research his options by meeting others who were in the throes of deciding to take their own lives – and to follow them to Switzerland where the non-profit group Dignitas would help to put them to permanent sleep. (It is illegal for an individual to assist another to die in Britain.) That he primarily spent his time with those determined to die rather than to carry on living, that he acknowledged but did not linger over the fact that, troublingly, a fifth of people go to Dignitas because they are "weary of life" rather than because they are terminally ill, was, one suspects, not so much a neglectful oversight (as some detractors would have it) as much as it was a measure of Pratchett's state of mind.

Peter Smedley, 71, struggled even to stand to greet the author, such was the vigour with which motor neurone disease was attacking his muscles. It's a "beastly, undignified business", said Smedley – and it was he who we were to see taking a barbiturate-based drink that would first send him to sleep and then to his death.

It was far from easy to watch, not least when, choking, Smedley asked for water – a request that was denied. Which answers complaints from some quarters that the programme made the process seem too much of an "easy" way out.

What's more, the impact of seeing a life drain away has been matched by the immediate impact the documentary has had in pushing the issue of assisted death back on to the national agenda. It has made us talk about something we would rather not, and has done so without being mawkish or strident.

As to whether it helped Pratchett come to a decision ... the implicit suggestion was that by the time his Alzheimer's has become so detrimental to his health that he no longer thinks life is worth living, no doctor in an assisted-death organisation would sign him off as having the faculties to decide that he wants to die. So he is left in something of a Catch-22: take his life before his disease makes him want to die – or accept that he must live with it.

More death came with the start of American series Castle, mercifully of the fictional kind. The wildly talented Nathan Fillion plays the eponymous crime author Richard Castle, who is swept up in a police investigation into a series of murders based on the kills in his books.

There really seems nothing new here: the plot is clichéd, but not quite as badly as some of the lines. ("Who are you?" asks a detective to a corpse.) It's impossibly glossy and occasionally cheesy. So why is it so watchable? Well, those clichés are so bad that it in fact turns into brilliant parody; some lines are absolute gold and Fillion is, simply, magnetic. He has the film-star looks but he's not afraid to come the fool, and, in the words of Stana Katic, who plays the detective-soon-surely-to-be-his-love-interest, he's "like a nine-year-old on a sugar rush". It's CSI meets Murder, She Wrote meets Californication without the fornication. For a Friday wind-down, it's pretty darn good.