Thursday’s “The Penguin,” Colin Farrell’s spin-off HBO series from “The Batman,” was in the works even as that 2022 hit movie was being made.

Originally seen in the campy ‘60s TV series by Burgess Meredith and then by Danny DeVito in Tim Burton’s “Batman Returns,” the disfigured Penguin — Oswald “Oz” Cobblepot – is, despite handicaps, a formidable Gotham City gangster.

Farrell, 48, transforms completely and is unrecognizable. Makeup artist Mike Marino, Oscar nominated for “The Batman,” reprised his duties here.

The design began with the movie’s writer-director Matt Reeves. “I knew Matt was an artist. We were both very geeky and fans of Batman,” Marino said at a “Penguin” press conference.

“He described Oz to be a sympathetic person similar to (the doomed brother) Fredo in ‘The Godfather.’ He mentioned ‘30s gangsters, ‘50s gangsters.

“I looked at penguins, to capture that subliminally in Colin’s face. I found a powerful brow in a penguin, a bird’s mouth, and created this person who doesn’t exist. Colin tried it the first time — and the walk came out and the voice came out.

“It took many months to fabricate, sculpt and develop so many things to make it move.”

“The first application was eight hours and everyone was taking their time,” Farrell recalled. “This was months before filming, there was no waiting crew. It was artists in a studio in Burbank.

“After we did the film, we got to three hours every day. We had our own trailer for the Penguin, nobody could come in. It was a great time for me. I’d shave, sit in the chair and then I’d think about the scene. It was seamless, never a drag.”

To create the Penguin’s character, “I had conversations with Matt about the psychology of Oz,” Farrell added.  “How paranoid he was and how he didn’t want to be seen as someone crippled.

“I’d done loads of thinking but when you put on the makeup, if you put something on that 100 percent changes everything you are, it will be different at first and strange.

“Six months before shooting ‘The Batman’ I looked at this creation. I was confused by it but it spoke to me so clearly with a threat of violence. And also, so sympathetic.”

Letting go?  “Taking the makeup off every day was helpful. It was 45-minute removal and by the end after being in it for 15 hours, I’m not (expletive) feeling sorry for it to go.

“The relief every day was like being reborn. At the end, he is so dark and so remorselessly cruel, I was glad to be done. I would go back to my hotel and watch Pixar films. I watched ‘Finding Nemo.’ ”

“The Penguin” streams on HBO/MAX Sept. 19