HOLLYWOOD — Robert Wagner gets a bit emotional explaining how he came up with the title of the memoir he wrote with Scott Eyman, “Pieces of My Heart.”

After receiving a rough manuscript of the autobiography, he recalls, he decided to read the chapter about Barbara Stanwyck to his wife, Jill St. John, and a friend. The veteran Stanwyck and Wagner had a secret four-year relationship in the 1950s when he was in his early 20s and she was in her mid-40s. “She was a wonderful woman,” Wagner says. “She was such a big part of my life.”

Wagner got so choked up during the reading that he couldn’t finish. He says their friend commented, “‘It must be very difficult to write a book. It must be very painful.’ I said, ‘Yes. It’s taken pieces of my heart.’ That’s how I got the title.”

Wagner’s book has plenty of stories about his stern, demanding father, his early years in Hollywood, his friendship with such legends as Spencer Tracy and David Niven — “you couldn’t ask for a better friend” — and his career ups and downs.

The crux of “Pieces of My Heart,” however, revolves around his first — and third — wife, Natalie Wood, who died in 1981. For the first time since her death, Wagner talks publicly about the fateful night. But there are no surprise revelations in his account; he reiterates what the coroner and police concluded: Wood’s drowning was accidental.

Wagner is 78. His daughters are grown, and he’s a grandfather. He remains handsome and charming.

Known as one of the nicest guys in Hollywood, the actor admits he’s astonished at the age range of his fans.

Wagner, however, said it was “very difficult, very painful” to write about Wood’s death.

“We were such a romantic couple, actually. The highlight of our lives was when we got back together. And then this horrible tragedy … good God.”

Wagner and Wood first married in 1957 and divorced five years later. Both went on to marry other people — and have children — only to get back together and remarry in 1972. They had their daughter Courtney in 1974.

In the fall of 1981, Wood was in Raleigh, N.C., making the sci-fi film “Brainstorm” with Christopher Walken. It was the first time Wagner and Wood had been apart since their remarriage.

About a month into filming, Wagner visited the set, where he noticed he didn’t have Wood’s full attention. He describes Wood as being “emotionally unfaithful.”

Did he think she was having an affair with Walken?

“No,” Wagner said quietly. “She was involved in the picture and involved in the work …”

Wagner and Wood had invited Walken to be their guest that Thanksgiving weekend on Santa Catalina Island, Calif., aboard their boat the Splendour. The evening of Nov. 29, the trio had dinner and drinks. They returned to the boat and continued to drink until a heated argument erupted between the two men. Wagner says it concerned how much of one’s personal life should be sacrificed in pursuit of one’s career; he was upset that Walken was advocating that Wood give all to her art, even at the expense of her husband and children.

Wood left to go to the master cabin’s bathroom. Wagner said he and Walken eventually calmed down and said good night. When he went to bed, he said, Wood wasn’t there.

It’s believed the dinghy had gotten loose and Wood came up on deck to tie it up.

“I have gone over it so many millions of times with people. Nobody heard anything.

“It’s in the book … but she had marks on her (body). All of those things lead to the belief that she had slipped and rolled into the water, which makes a lot of sense because the boat — when they found it, it hadn’t been started and the oars were all in the same position. There was no evidence that she tried to get in it.”

These days, Wagner and St. John, who have been married 18 years, live in Aspen, Colo., and keep a pied-a-terre in L.A.

He has no plans to retire from acting, though he won’t be returning to his guest role as Holland Taylor’s paramour on “Two and a Half Men.”

“I loved doing that show, but they decided to get the guest artists out of the show and go back to the original (format),” he said.

So Wagner’s character was killed off on his wedding day — suffering a heart attack after having sex with his younger lover (Jenny McCarthy) right after marrying another woman. “I was very disappointed about leaving the show, but what a way to go — to have Jenny McCarthy make love to me in the most marvelous ways,” he said with a wry smile.