A producer who collaborated on the 2015 film “Being Charlie,” directed by Rob Reiner and his son Nick Reiner, is breaking his silence following the horrific deaths of the renowned director and his wife, Michele Reiner.

Douglas Shaffer shared with Page Six that he feels “mixed emotions” upon hearing the news that the couple’s troubled son has been arrested and charged with the brutal stabbings that occurred at their Brentwood, Los Angeles residence on Sunday.

“I feel a deep sadness because it’s a tragic event where a father was trying to help his son, and somehow everything went catastrophically wrong,” Shaffer commented.

“This is a classic Hollywood tragedy. It’s absolutely sickening,” he continued.

“Rob was a monumental figure in Hollywood, truly a legend,” he remarked. “With his father, Carl Reiner, also being a Hollywood icon, it’s devastating to see their legacy end in such a tragic manner.”

Shaffer continued, “Another part of me is angry. Nick… if there were mental issues involved, that’s one thing. I don’t know what the extent that is, if he was showing signs of schizophrenia or whatever. But if he was just in a drug-induced rage, I’m so angry about that. He had a good support system with his family and his father, and all they tried to do was help him. And to somehow blame his father and his mother for all that, and the act that he did is just absolutely sickening to me.”

Shaffer was an executive producer on “Being Charlie,” which centers on an 18-year-old addict son of a movie star running for Congress.

The film was written by Nick, 32, and a friend from rehab who were inspired by their experiences with addiction. Rob directed the film and also co-produced it.

Shaffer told Page Six that while Nick — who has been in and out of drug rehab since age 15 — may be dealing with “underlying medical issues,” he isn’t “sympathetic” towards Rob’s son for allegedly committing the killings.

“It’s such a grotesque act to do that,” Shaffer noted. “I just have trouble even comprehending that. It’s just so sickening to me.”

Shaffer, who has a son that is non-verbal autistic, broke down in tears while relating to Rob’s struggles as a father.

“That’s where it becomes emotional for me is because I can see that plight of seeing your son who is struggling .. It’s hard because you want to do anything for them, and when I think about what Rob tried to do for his kid and then for that to happen is shocking,” Shaffer said while crying. “It’s more from a father’s point of view.”

“The senselessness of it… I just can’t picture that,” an emotional Shaffer added. “It’s hard to think about because I try to relate it to my kid. How heartbreaking this is for the family and everyone involved. It’s just awful for that to happen.”

Shaffer also acknowledged that Rob made the semi-autobiographical film “as a love letter” to Nick.

“He was so proud of Nick during the making of the movie, and then when he was promoting the movie, he was so proud of him,” Shaffer recalled. “He was like a doting dad. He said during a private screening the best experience of his working career was working with his son. He wouldn’t change it for the world.”

“He was doing everything to help him,” Shaffer added of the “When Harry Met Sally” filmmaker. “There were tactics they had tried early on with Nick to get him into rehab. It wasn’t working. He kind of switched tactics. And I think he was really proud of his son.”

Shaffer went on, “But he was also trying hard as a father to understand Nick’s plate, so to speak. Like what that is to be addicted to drugs. How certain things like rehab you think are going to work don’t work. Because Rob was a very no-nonsense type of guy. But he was very open to any type of conversation. I think he wanted to learn more about what his son went through. The movie was almost therapeutic in that way for him and Nick. And I know he didn’t like reliving it because it’s very autobiographical. So I think they both were kind of reliving their experiences when making the movie.”

From his perspective, Rob and Nick healed their complicated relationship when they made “Being Charlie” together.

“It seemed like it was therapy. They got it all out there,” Shaffer said. “They’re very honest about their relationship. Rob did not hide anything about how he looked at addiction when it was happening in the family. I think he took more of like, ‘Just do rehab.’ And then he started to understand some of these things don’t work. And I think he felt guilty because he was pushing certain things that maybe he shouldn’t have been pushing. But again, it seemed to be a healing experience. I had no idea any of these things were happening afterwards. I thought Nick was totally on the up and up.”

When “Being Charlie” came out, Rob openly expressed regret at how he handled his son’s addiction battle.

“When Nick would tell us that it wasn’t working for him, we wouldn’t listen. We were desperate and because the people had diplomas on their wall, we listened to them when we should have been listening to our son,” Rob said during a 2015 interview with the LA Times.

Since Rob and Michele’s murders, new details have come to light regarding Nick’s behavior and his relationship with his parents.

The trio allegedly had a massive fight at Conan O’Brien’s Christmas party the night before the Reiners were killed — possibly because Nick was back on drugs and refusing yet another go at treatment.

“Rob had been telling people that they’re scared for Nick and scared that his mental state was deteriorating,” a lifelong family friend, who lives near the Reiner home, told The Post.

Rob and Nick had another public fight months earlier at a restaurant in Santa Monica Canyon, a worker confirmed to The Post.

In addition, an insider told Page Six Monday that Nick “really resented his dad” and “hated himself for not being as talented, prolific or beloved as his dad or grandad.”