Paul Kaye plays Pratchett in Back in Black, based on Pratchett's unfinished autobiography; it will air on Saturday.
The doc covers the frustrations, discrimination and discouragement that Pratchett encountered as a working class pupil with a variety of speech impediments, and on what Neil Gaiman called Pratchett's 'quiet rage', which fuelled him to literary stardom and enabled him to write seven novels even as Alzheimer's stole his mind.
The irreverent trailer hints at a programme that will treat Pratchett with the kind of anger and compassion he brought to his own work and life.
On his Alzheimer's, Pratchett wrote: "On the first day of my journalistic career I saw my first corpse – some unfortunate chap fell down a hole in a farm and drowned in pig shit. All I can say is that, compared with his horrific demise, Alzheimer's is a walk in the park. Except with Alzheimer's my park keeps changing.
"The trees get up and walk over there, the benches go missing and the paths seem to be unwinding into particularly vindictive serpents.
"I always dreamt that when I died I would be sat in a deckchair with a glass of brandy listening to Thomas Tallis on the iPod. But I had Alzheimer's, so I forgot all about that."
While his novels were popular with readers, Pratchett's work was often dismissed by critics. He wrote that the feeling of "somehow being inferior" picked up during his school days had stayed with him and was "hard to shake off". But his anger, he said, carried him "quite a long way".
During his later years, the writer campaigned for terminally ill patients to have a right to die, and featured in a documentary in which he followed a man with motor neurone disease to the Swiss clinic Dignitas to watch him take a lethal dose of barbiturates.
Terry Pratchett docudrama reveals moment author realised he was 'dead'
[Nadia Khomami/The Guardian]