When his own time comes to die, the author Sir Terry Pratchett has said, he would like to be sitting on a chair on the lawn at his home, with a brandy in one hand and a glass of life-ending chemicals in the other. "And with Thomas Tallis on my iPod, I would shake hands with Death."

The question of his own death – and the right to choose its timing and manner – has become more pressing for 63-year-old Pratchett since his diagnosis in 2007 with early-onset Alzheimer's.

Now the vastly successful fantasy writer has made a documentary about the subject of assisted dying, for which he travelled to Switzerland to film the final moments of Peter, a British man suffering from motor neurone disease who had chosen to end his life at the Dignitas clinic.

In an emotional interview with the Radio Times, published on Tuesday, Pratchett says he felt as if he was "spinning" in the moments after Peter's death. "Something was saying, 'A man is dead … that's a bad thing,' but … he had an incurable disease that was dragging him down, so he's decided of his own free will to leave before he was dragged. So it's not a bad thing.

"So then is it a good thing? And you are trying to resolve these things. Because it is a good thing, I think, that in those circumstances Peter got what he wanted – a good death."

It will not be the first time such a death has been broadcast: in 2008 a Sky documentary recorded the dying moments of another motor neurone sufferer, Craig Ewart, 59, also at Dignitas. But Pratchett's film, which also focuses on the cases of two other men facing similar choices, is likely to reopen the debate over the legal and moral constraints on assisted dying. Pratchett, the author of the Discworld series of fantasy novels, who was knighted in 2009, will be glad if it does. He backs assisted death, he tells the magazine, "because if someone knows they can die when they want to, they can treasure every day. They can think, 'The grandchildren are coming over tomorrow' or 'It's nearly Christmas so I'll leave it till the new year … it's a bit painful but we can hang in there.' So someone is doing an incredibly human thing, something that no animal can ever do – actually controlling, if not forestalling, their own death – and getting some pride out of that, I suspect.

"I do not wish to have to prescribe to Britain what it wants," he says. "I would like to see in the UK an examination of the methods of assisted dying so that we may consider what is best. You know, what suits the British." In a Richard Dimbleby lecture last year, Pratchett declared his intention to "live my life as ever to the full" and die on his own terms. "This seems to me quite a reasonable and sensible decision for someone with a serious, incurable and debilitating disease to elect for a medically-assisted death by appointment."

He suggested the creation of an independent tribunal of experienced and "wise" figures who could act "for the good of society as well as that of the applicant" in deciding whether they should be allowed to choose to hasten their own deaths. Owing to his illness, the lecture was read on Pratchett's behalf by his friend, the actor Tony Robinson.

"If I knew that I could die at any time I wanted," Pratchett said through Robinson, "then suddenly every day would be as precious as a million pounds. If I knew that I could die, I would live. My life, my death, my choice."

Charlotte Moore, the BBC's commissioning editor for documentaries, tells the Radio Times that the scenes in which Peter dies were "extremely powerful and challenging … but above all … honest. To gloss over Peter's final moments would be to do a disservice to Peter, to Terry and to the viewer. We have a responsibility to tell the story in its entirety. How can we do this if we shy away from the crux of the story, difficult as this may be?"

Terry Pratchett: Choosing to Die is on BBC2 on Monday 13 June at 9pm