When mulling over who could do justice to the voice of Satan on “Good Omens,” the only name that popped up in Neil Gaiman’s mind was Benedict Cumberbatch — but not for the reason you might think.

“It was from the point of view of going, ‘OK you’re in a show in which Frances McDormand is God,'” Gaiman tells Variety. “And we need somebody who can act — who is not going to be just blown off the screen by the idea of Frances McDormand. I was sitting there going, ‘Who has the who has the vocal range?'”

In Amazon’s six-part series adaption of Gaiman and Terry Pratchett’s novel of the same name, Satan is a 500-foot tall figure who certainly looks terrifying, but Gaiman says he actually wanted to depict the human side of the creature. So when Satan appears in the sixth and final episode, he’s not wreaking havoc, instead, he’s having an intimate conversation with his son (Sam Taylor Buck), the Antichrist, after being absent from most of the child’s life. And on a show with such formidable actors like Michael Sheen, David Tennant and McDormand, Gaiman knew he needed the right performer to imbue Satan with much-needed nuance.

Originally, Gaiman, who served as showrunner and wrote every episode of the show, shares he read Satan’s lines but very quickly knew they needed “something much better.” He then thought of Cumberbatch. The author had seen him previously perform at a London charity, where he read famous letters throughout history.

“I just watched him act and went, ‘This is magic. This is absolutely amazing,'” he says. “And so when I started thinking, ‘Okay, who can I find? Who could give me this performance in a small enough space?’, Benedict was simply my first choice.”

Luckily, “Good Omens” director Douglas McKinnon had worked with Cumberbatch on BBC’s “Sherlock” and had his contact information. So Gaiman took a chance, shot the actor an email, and the next thing he knew, Cumberbatch was in the recording studio.

“A soon as Benedict started making the lines his own, the whole thing came to life. Suddenly, it was no longer a monster movie,” he says. “Suddenly, it felt like ‘Good Omens’ again. You had somebody who might be 500-foot high, that was basically an absent father unable to deal with a rebellious kid.”

Then, they gave the audio and visual footage to VFX supervisor Jean-Claude Deguara and his team at Milk VFX to create a CG Satan that had some of Cumberbatch’s features and movements, to make the character feel fully fleshed out.

Even though Satan never actually appears in Gaiman’s novel, the writer-producer says he knew that for the adaptation the audience would have to see the character. Additionally, he created a whole new storyline after the Satan scene that also wasn’t featured in the book, to keep fans on their toes.

“It’s going to keep you watching and engaged, I hope, until the last seconds of Episode 6. That to me was important going, ‘Ok, I will let be the big Satan appearing climax happen where it would’ve happened in the book. But instead, I’m going to introduce another plot,'” he says. “And partly, it was just the knowledge that so many millions of people have read the novel, that you want to surprise them — you don’t want them all just being cocky and going, ‘I know what’s going to happen all the way through.'”

“Good Omens” is streaming now on Amazon Prime Video.