There was an empty seat in the front row when Good Omens had its world premiere in London on Tuesday.

But that's not because organisers had trouble filling the gigantic (and newly reopened) Odeon in Leicester Square - quite the opposite, the event was packed out.

In fact, a seat was deliberately kept vacant for Terry Pratchett, the co-writer of the original novel, who died in 2015.

As a tribute, his trademark hat was placed in the front row as the premiere got under way.

As Peter White noted in Deadline, it's highly unusual for a TV series such as Good Omens to "receive a glitzy world premiere in Leicester Square" as that's "a feat usually reserved for big-budget superhero movies".

The new show, which will launch on Amazon Prime on Friday, has been adapted for the small screen by Neil Gaiman, who co-wrote the 1990 novel with Pratchett.

The fantasy series sees an angel and a demon (played by Michael Sheen and David Tennant) team up to save the world as the end of time draws closer.

While a hugely popular novel may seem fertile ground for a screen transfer, a string of producers and writers have refused to touch it over the last three decades.

"Once upon a time, Good Omens was considered unadaptable," wrote Flora Carr in the Radio Times.

"Neil Gaiman and Terry Pratchett's sprawling fantasy novel was notorious within the film and TV industries. Screenwriters turned their noses up at the project, and various attempts over the years to bring page to screen ended in disappointment."

But, according to Gaiman, one of Pratchett's last requests was that the novel be adapted for the screen.

"[Pratchett said] 'You have to make it into television because I want to see it before the lights go out,'" explained Gaiman at the premiere.

"I said OK, figuring I had six or seven years of Terry left. And then he died, which suddenly turned into a last request."

Gaiman took on the task of adapting the story himself - giving the project a stamp of approval that helped it attract some stellar names to the cast.

 Author Terry Pratchett died in 2015, eight years after he was diagnosed with Alzheimer's

"It helped so much having Neil Gaiman being the showrunner," Sheen told BBC Radio 2 ahead of the premiere.

"He was on set every day, working alongside Douglas Mackinnon, who directed it, and was at the heart of all the creative decisions, which gives you a lot of confidence. He's not done that before."

Similarly, fellow cast member Adria Arjona, who plays Anathema Device, told Mail Online: "I think that this has been the job where I've felt the most comfortable because I've had the source in front of me.

"If I had a question, I could just go to him, and that to me was really helpful because the acting process sometimes can be so lonely in a way. But here it was such a collaboration."

The rest of the cast includes Miranda Richardson and Jon Hamm, with Frances McDormand as the voice of God and Benedict Cumberbatch as the voice of Satan.

Good Omens premiere in pictures

Miranda Richardson

Yusuf Gatewood

Jon Hamm

Josie Lawrence

Neil Gaiman

Perhaps surprisingly, Gaiman said dividing the novel into six hour-long episodes was "definitely the simplest part".

He said: "I was very practical, I sat down with the novel, the edition I had was 300 pages long, so I put post-it notes in every 50 pages, decided that was what was happening in all those episodes."

But, he added: "Throughout it all, I kept wishing that Terry Pratchett was there. Whenever I got stuck, I wanted to call Terry and say, 'What do I do now?' And whenever I did something clever, I wanted to call him and say, 'I did it, I figured it out!'"

The show has had mixed reviews from critics so far - but many credit the pairing of Sheen and Tennant with carrying the series.

"It's a lot, and sometimes the pace is more exhausting than bracing," Judy Berman wrote in Time. "At the same time, the show's underlying ideas about tribalism and friendship are pretty commonplace.

"Still, Tennant and Sheen make an ideal buddy-comedy duo; their banter does justice to Gaiman and the late Pratchett's witty prose.

"Like all maximalist TV, Good Omens promises to be polarizing. It isn't my idea of heaven, but your paradise may vary."

Writing in IndieWire, Ben Travers said: "Divine turns by Michael Sheen and David Tennant get lost in an overloaded plot of diminishing returns.

"Douglas Mackinnon's direction makes the most of lush environments and a farcical tone, but the meandering editing doesn't always do the scenes justice.

"McDormand's narration, while amusing, can be a crutch, and there are distinct mistakes in timing, whether it's when a song kicks in or when shots start and end."

"The infantilist tone recalls the Harry Potter universe," noted Suzi Feay in the Financial Times.

"Like human history, this goes on a bit but is enlivened by highly entertaining patches."

In her Radio Times review, Flora Carr noted that, if anything, efforts to stay true to the book were a hindrance to the flow of the TV series.

"Reams of dialogue are almost word-for-word during episode one, to the extent that there are certain moments and scenes where one feels that the show's pace has been sacrificed in favour of preserving the 'voice' of the book," she said.

But speaking at the premiere, Douglas Mackinnon stressed the importance of doing justice to Gaiman and Pratchett's original text.

"We had a very good budget, but we didn't have an unlimited budget, and oddly the training I'd got doing shows like Sherlock and Doctor Who came into force all the time," he said.

"The challenge was keeping the story faithful to the book, to Terry, to the scripts that Neil had written, and just getting the vision out - that was the key to it all."