The late Terry Pratchett would have been “over the moon” at the “dream” casting of David Tennant as the demon Crowley in the forthcoming adaptation of Good Omens, according to the Discworld author’s long-time assistant Rob Wilkins.

Amazon confirmed that Michael Sheen will play the angel Aziraphale, and Tennant will take on the role of Crowley, in Amazon Studios’ six-episode adaptation next year. Co-authored by Pratchett and Neil Gaiman, the 1990 fantasy bestseller Good Omens tells of Crowley and Aziraphale’s attempts to prevent the apocalypse, following the birth of the antichrist, Adam, in Lower Tadfield, Oxfordshire.

“I swear to god, Terry would be absolutely over the moon. We’d already talked about our dream Aziraphale and Crowley, and David Tennant was there,” Wilkins told the Guardian. “I can’t begin to tell you what a fan he was of David and Michael … He’d be looking down now and would be so incredibly pleased.”

Pratchett had particularly enjoyed Tennant’s role as Doctor Who, Wilkins added. “Terry didn’t use magic as a get out of jail free card in his writing, and he felt that after David, Doctor Who used the sonic screwdriver as a way to get himself out of trouble. And Terry would groan, as he would never use magic in the same way.”

Gaiman, who scripted all six episodes of the forthcoming series, which is co-produced with BBC Studios, wrote on Twitter after learning of the casting: “You go down to the bottom of the garden to write, and leave your phone behind, and then something like this happens.”

The novelist, who only agreed to adapt Good Omens after reading a letter sent posthumously to him by Pratchett giving him his blessing to do so, also reminded Variety that the book had two authors after the magazine’s headline referred to the book as “Neil Gaiman’s Good Omens”.

“Psst. Neil Gaiman and Terry Pratchett’s Good Omens,” wrote Gaiman, while Wilkins made the same point: “My job as executive producer on #GoodOmens is primarily to remind the world the book had TWO FECKING AUTHORS.”

My job as Executive Producer on #GoodOmens is primarily to remind the world the book had TWO FECKING AUTHORS @Variety. — Terry Pratchett (@terryandrob) August 14, 2017

Pratchett died in 2015 at the age of 66, leaving behind him more than 70 books. “My job is to ensure the world speaks his name – that’s what I’ve always said since Terry died,” said Wilkins, adding that he had recently joined a writers’ meeting at Narrativia, the independent production company launched by Pratchett five years ago. “One of the writers stood up and said that of all the dead writers there have ever been, Terry Pratchett is the most alive. For the next hour of the meeting, I was so filled up with those words, and I have kept them with me.”

Wilkins said today that Gaiman’s scripts would make fans “so happy”. “He has completely blown us away,” he said. “They are magnificent … This is the Good Omens of my dreams. When you have six hours to tell the story, with a movie budget, it is a dream come true. And Neil has taken the words from the page on to the screen so beautifully.”

Amazon confirmed the casting in a statement, after it was first reported in Variety. In the statement, Sheen said that Good Omens had been one of his favourite stories since he first read it as a teenager. “To be part of the team entrusted with bringing it alive on screen is a bit of a dream come true. To work alongside Neil, who I think is one of the greatest storytellers of all time, is incredibly exciting. And, just like the rest of the world, I’m a huge fan of David’s so I relish trying to save it with him,” he said.

Tennant called it “a dream team and a dream gig”, adding: “I am very excited that I’ve been asked to be part of this amazing project. Bring on the apocalypse!”

Good Omens will air on Amazon Prime in 2019 and will be broadcast on BBC Two at a later date.