Ettinger, 34, plays teen/college-age identical twins Dominick and Thomas Birdsey in the acclaimed HBO drama starring Mark Ruffalo (as the older brothers).

As the series progresses, Thomas’ paranoid schizophrenia manifests itself when the brothers get to college; Dominick, who fiercely loves and protects his brother, can only look on helplessly.

“My brother, who’s 13 years older than me, started to get symptoms [of schizophrenia] when he was in his early 20s and I was around 9,” says Ettinger, who grew up in nearby Fair Lawn, NJ. “The age gap what it was between us, I didn’t understand what was happening.

“He’s doing great now and he’s been watching [the series] and it’s really moving him,” he says. “My own dad, who’s not the most outwardly emotional person, has called me after every episode and he’s really emotional. He repressed a lot of his own feelings and related to Dominick’s frustration and anger toward Thomas.

“Yes, Dominick can act like an a–hole at times, but he’s trying to cope with the skills and tools he has,” he says. “He’s got so much weight on his shoulders — and I’m happy we didn’t sugarcoat that.”

Ettinger (“One Dollar,” “The Evening Hour”) says his personal history was his motivating factor in auditioning for the roles.

“It wasn’t to score a job with cool people,” he says. “I felt inspired to represent [the brothers] correctly and subtly and not do the crazy cliche thing, to find the nuance. When I got the audition, a good friend of mine, who had already auditioned for the young Dessa [Dominick’s future wife], e-mailed me and said, ‘You’re going to get the role.’ I felt that the universe was laying something out for me.”

Ettinger shot his dual roles while Ruffalo — who filmed his part as the 40-ish Dominick first — took time off to gain weight for his portrayal of the older, heavily medicated Thomas, who’s spent two decades in Connecticut psychiatric facilities.

“I was the first one to play Thomas, so that was interesting,” Ettinger says. “Mark and I had to collaborate on that. There was a great night where we met in a diner on the Upper West Side and walked all around the city. I kind of needed to get his blessing because I had this idea of how to play Thomas and luckily there were many kindred feelings between us.

“We joined forces on that one.”

Ettinger says that filming both roles simultaneously was extremely challenging.

“We tried everything that first day,” he says. “We had a double there and recorded his voice and played it back to me and that didn’t work. I played Dominick first, to try to get the skeleton of the scene — I would do scenes that made me a little frustrated, angry and repressed — and then I would release that filter [for Thomas]. We ended up improvising a lot.”

Ettinger says the scene that really helped him “get” the brothers’ emotional essence is when Thomas asks Dominic what it’s like to sleep with a woman.

“I got so emotional playing Thomas in that scene, where Dominick is telling him what sex is like and Thomas knows he’ll never experience anything like that,” he says. “That happened in the moment and it wasn’t planned. Luckily they caught it on camera.”

And, Ettinger says, he’s been hearing from people all around the world since the series premiered.

“The response of people who’ve dealt with these issues of survival and are struggling to keep their heads above water has been so special,” he says. “I didn’t know if people would tune out because it’s too much to handle, but they’re relating to it and connecting through the pain and emotion.

“It’s cool to see.”