The This Is England '86 actor plays Neil in the supernatural drama, which begins tonight on BBC Three.
"They sent the script through and it was quite wild," he told CultBox. "I love that, [because] there's no limits to where this goes. Each time you turn the script it goes off in another direction."
Harris added that he was keen to reunite with series creator Jack Thorne, who previously co-wrote This Is England '86 with Shane Meadows.
"I've not worked in the sci-fi/fantasy genre before," he explained. "I got told it was [written] by Jack Thorne [and] I was actually working on This Is England when I got the script.
"The second I heard it was by Jack Thorne, it was just a gut instinct - 'I'm interested'."
The Fades also stars Iain De Caestecker, Daniel Kaluuya and Lily Loveless, and has been directed by Farren Blackburn, who is currently filming this year's Doctor Who Christmas special.
The series begins tonight at 9pm on BBC Three.
> The Fades: What's BBC Three's new drama all about?
Watch Digital Spy's interview with the cast of The Fades below: