Hal Holbrook didn’t require real-life details to craft his iconic role in the 1976 drama “All the President’s Men.” He had his own bloodhound-keen approach to finding the performance: Follow the morality.

Holbrook, who died Jan. 23 at age 95, played Deep Throat, the secret “garage freak” of a source to Washington Post reporter Bob Woodward (Robert Redford) during the Watergate scandal. In 2005, W. Mark Felt, the former No. 2 official at the FBI, revealed himself to be Deep Throat — ending three decades of public speculation.

“It was a brilliant performance,” Woodward said Tuesday, upon hearing of Holbrook’s death. “He captured the intensity and the sense of Mark Felt’s distance,” as Felt weighed the act of helping amid his internal conflict.

Yet Holbrook captured the character without knowing Deep Throat’s true identity. Woodward met Holbrook briefly and notes that the actor never asked him for real-world specifics.

In 2002, I ended an interview with Holbrook by asking him whether he had any inkling as to who Deep Throat was. He suggested that the filmmakers may have had their own guesses — but he did not.

“As Redford and [director Alan J.] Pakula talked it over before shooting in that garage,” drawled Holbrook, laughing lightly in his reminiscence, “I stood there, literally and figuratively in the dark.”

Redford was friends with Holbrook, and told The Post on Tuesday in an email through his representative that this bond “thankfully provided the opportunity for him to play Deep Throat” in what the star calls “a memorable and iconic performance.”

Holbrook portrayed numerous historic figures throughout his career — even embodying Mark Twain longer than Samuel Clemens played him. The actor said he spent part of his career escaping Twain’s long shadow. By stepping into the shrouded role of Deep Throat, Holbrook found magic in exploring shades of intrigue.

“The backstory I created for him was based on a question of morality,” Holbrook told the New York Times in 2005. “This man was breaking the rules of Washington by telling on the president. I thought, ‘What kind of man might do that?’ ”

The actor concluded that Deep Throat was “faced with the choice of loyalty to his president or loyalty to his country. He chose his country. But it was so distasteful to him because he was a person of character, and here he was, passing information along in a garage.”

What Holbrook had to convey, acting opposite Redford’s increasingly exasperated reporter, Woodward said, was “the exquisite ballet of Mark Felt.” The character varies between expressions of self-centered control (“I have to do this my own way”) and eruptive anger (“You’ve done worse than let [President Nixon’s chief of staff H.R.] Haldeman get away; you’ve got people feeling sorry for him”).

The Emmy- and Tony-winning performer knew the importance of capturing “that ambiguity of somebody saying, ‘Oh yeah, I’m going to help you, but let’s meet in an underground parking garage at 3 a.m.,’ ” Woodward said. The character’s attitude was: “I’m going to help, but I don’t want anyone to know I’m doing this. I don’t want anyone to ever know I’m doing this.”

Eventually, Woodward recalled, Felt reached a point where he pointed toward the Watergate scandal’s scope because the reporter wasn’t fully grasping it. “He was almost shaking me to get it overall.”

The scene where Deep Throat finally says “Get out your notebook” was the interaction that spurred Woodward and collaborator Carl Bernstein to see the bigger picture. The 1972 break-in at Democratic headquarters in the Watergate hotel was no mere burglary, Deep Throat was saying, but “a whole series of operations — campaign sabotage and espionage,” Woodward recounted.

One of Holbrook’s most famous phrases in the film became “follow the money.” Woodward said the line was invented by the filmmakers — “It’s not in the book, it’s not in our notes” — yet still reflected what Felt was saying without saying it.

Holbrook told me part of his technique in playing Twain was holding the long pauses — a gift for restraint and silence that he brought to Deep Throat. Woodward also praises the art of the long pause. “Let the silence suck out the truth,” he said he teaches to journalism students.

Holbrook’s brilliance also lies in his appetite for creative discovery. He was “an actor on a quest,” Woodward said. “He was not passive. It was always: ‘What have we got here?’ ”

Woodward paused himself while weighing Holbrook’s ability to inhabit a man the actor knew nothing about.

“One of his genius parts, isn’t it?”

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