Neil Gaiman has spent two years working on the TV adaptation of his and Terry Pratchett’s novel “Good Omens” and told an international TV crowd this week he dedicated himself to the task to honor a request from his friend and co-writer, who died in 2015.

“I didn’t in any real sense of the word have two years of my life to give up to make this, so I wouldn’t have done it if Terry hadn’t died and given me this as his last request,” Gaiman said. He added “Terry’s wish trumped everything, so we were making it for him” and teased that “the very last thing that happens on the final screen of episode six is just for Terry.”

The series has an all-star cast that includes Michael Sheen as earth-dwelling angel Aziraphale and David Tennant as his demon counterpart Crowley. Jon Hamm is the angel Gabriel and Benedict Cumberbatch is the voice of Satan.

Gaiman said he missed being able to call Pratchett, as he had during the writing of the book, to talk through the project as it took life. He said he had used a lot of material that the pair had discussed as going into possible sequels to the “Good Omens” novel in the TV series. “That stuff allowed us to actually solve some of the problems involved in putting it on television – the fact that we have Jon Hamm as the impossibly irritating angel Gabriel is something from the sequel we slid back in – [as was] the relationship between heaven and hell, where we keep going back and forth between these two appalling places,” he said.

Gaiman was speaking at a “Good Omens” session at the BBC Studios Showcase event in Liverpool. He loomed large over proceedings. Joining via a video link from New York, the writer appeared on a sizable video screen that hung above director Douglas Mackinnon (“Sherlock”), and exec producers Rob Wilkins, a longtime collaborator with Pratchett, and BBC Studios’ Chris Sussman, who were there in the flesh.

Wilkins noted that “over the years several very, very clever, very talented writers have tried and failed to put ‘Good Omens’ onto the screen – it just didn’t work” but that the stars aligned when Gaiman came on board to adapt the series.

Amazon will launch the show internationally in May and the BBC has a free-TV window in the U.K. BBC Studios is producing with Narrativia and The Blank Corporation. “It’s the only time in history where all the factors could come together, where we were able to make a 6-hour movie for TV,” Wilkins said.

The series defies fitting into a single genre and will be “delightful and baffling” Gaiman promised. “It isn’t like anything else,” he said. “The problem that we keep having when people say ‘what’s it like’ is they want us to compare it to something, and we don’t have anything to compare it to.”