Chandler Riggs in “A Million Little Things.ABC

Chandler Riggs was 10 years old when TV viewers met him as Carl Grimes on “The Walking Dead” — an award-winning role he played for seven years before Carl, already minus an eye, was killed off halfway through Season 8.

Riggs is back in prime time on Season 2 of “A Million Little Things” (9 p.m. Thursday) as the enigmatic Patrick “P.J” Nelson — convinced he’s the biological son of series ghost Jon Dixon (Ron Livingston), whose suicide in the series opener is the thematic motif around which ABC’s drama is framed.

Riggs didn’t watch Season 1 of “A Million Little Things” and says he was totally in the dark when he was called in to audition for the role of P.J.

“I was given absolutely no information about the character, only that he was in his early-to-mid 20s, had a troubled past and finds a script in the hospital,” he says. “My audition scene was the scene in the hospital with [co-star] Romany [Malco, who plays Rome Howard]. Maybe [P.J.] was in the hospital because his dad was in a car accident? I had no idea.”

Riggs, who’s also a DJ and creates “cinematic electronic music” — he cites Oscar-winning German composer Hans Zimmer as a major influence — says he already knows if Jon is really P.J.’s dad.

“Over the next few episodes [P.J.] is going to still be agitated about finding out the [DNA] test results and figuring out who his father is,” he says. “At the same time, he’s kind of being embraced by Rome and the other characters and … I’m trying to think of how I can say this … he’s going to start to kind of jump to conclusions a bit and get ahead of himself, which might backfire. We’ll see.”

Despite leaving “The Walking Dead” — his final episode aired in February 2018 — Riggs says he’s still keeps tabs on the series (renewed last month for an 11th season). “I love the show and I’ve been incredibly impressed by these last couple of seasons and can’t wait to see more,” he says. “It’s so interesting because it’s deviated so far from the comics. I’m an avid fan of the comics, as well, so I just have no idea where the show will go from here.”

He does say, though, that the vibe on “A Million Little Things” is much different than on “The Walking Dead.”

“It’s a lot lighter, that’s for sure,” he says. “We joke around a lot more and it’s definitely a much lighter environment in general. I didn’t realize how dark ‘The Walking Dead’ was … there were so many hefty, emotional scenes that weren’t easy to snap out of.

“For ‘A Million Little Things,’ it’s a walk in the park compared to that,” he says. “It makes everything a lot of fun.”