Greg Nicotero shares the secret to a new breed of walker.

The sight of Daryl and Carol together in the season 2 trailer for The Walking Dead: Daryl Dixon — The Book of Carol is certainly a striking one for fans. To see these two beloved friends back in the same frame together after two years apart had the fanbase abuzz. But there was another shot in that trailer that also lit up the fanbase — almost literally.

That’s because The Book of Carol is also opening the book on another type of walker we have yet to see on the franchise — bioluminescent zombies! And for the man who oversees all the undead on the show, executive producer Greg Nicotero, it was about elevating the work he and his team have been doing for 14 years. Even in scenes where they are not making what he describes as “giant advances” when it comes to walkers, Nicotero still strives to provide “a much more interesting take on” the flesh-eaters still roaming the apocalyptic wasteland.

Bioluminescent zombie on ‘The Walking Dead: Daryl Dixon — The Book of Carol’.AMC

So how did they create these new bioluminescent “hungry ones”? “During the strike, I came back to the States last year, and we were talking about the bioluminescent zombies,” Nictotero tells Entertainment Weekly. “And 70 percent of that was practical effects. We had LED lights, and we also had this black light wildfire system where we would paint the backside of silicone prosthetics with a black light-activated bioluminescent paint. And if you watch the outtakes of that scene, you’ll literally see my team standing there with black lights right on the zombie’s faces.”

The makeup and visual effects teams then worked together to create the final look. “A lot of what VFX did was augment, by taking a lot of the bioluminescence and spreading it up the walls of the cave.”

But it turns out they could only film these special zombies for two minutes at a time. “We did stuff where we were able to make the inside of the mouth glow,” Nicotero reveals. “So a lot of it was just hiding stuff, not only in the wardrobe, but then underneath the skin, painting the silicone appliances from underneath, painting some stuff on top. And then you knew that once you would charge it right before the take, you would have probably 120 seconds of really strong interactive light before it started to fade. So there was a lot of running in, and charging it, and coming out, and then VFX just sort of amped it up.”

Bioluminescent zombie attack on ‘The Walking Dead: Daryl Dixon — The Book of Carol’.AMC

Speaking of being amped up, that brings us to another group of the undead who will once again play a key role on the show. Those would be the experimented-on zombies that have been injected with a mystery substance turning them into super-fast walkers — also known as “ampers.”

“Now we have a better indication in season 2 of how these ampers play a really important part in Genet’s quest for power,” Nicotero says. “We wanted to make sure that they looked different, so we went with black contact lenses.”

That idea for the rest of the ampers’ look came from a cult classic film from the 1990s that the horror makeup guru worked on, as well as another touchstone from the genre. “The inspiration was some stuff that we did on The Faculty, and then Scanners where you see these veins kind of traveling underneath the skin. And that was something that I really, really wanted to do, which was sort of redefine and play into one thing that we never play into on The Walking Dead, which is the idea that these zombies are fast.”

Emmanuel Guimier/AMC

While we’ve seen both fast and slow zombies over the years in the genre, this was The Walking Dead’s first foray into a speedier breed of the undead. “The choice of fast or slow was always based on tradition,” Nicotero says, citing the grandfather of the genre (whom he worked with on the original Day of the Dead) that preferred a slower, lumbering threat. “The DNA comes from George Romero. But I think with the amped up zombies, we leaned much more into 28 Days Later trained to an Asian horror vibe. Trying to keep it so that it still feels like it belongs in our world, but pays homage and doesn’t feel out of place, was definitely challenging.”

That challenge will be on display when The Walking Dead: Daryl Dixon — The Book of Carol premieres Sept. 29 on AMC. In the meantime. check out our big cover story on how the season will connect back to the characters’ first meeting ever.