He and Lenihan, an expert in underwater archeology, met at a dive shop in Sante Fe, N.M. They share an interest in scuba diving and have taken several vacations together.
They have lunch together often and discuss new books they've read. More than once, they've said they could write a better book than many of the new releases. And eventually, they agreed to try it.
On Hackman's land in New Mexico, there's a trail that winds down a mountain - and through time.
"Almost without exception, every time I walk this path, I think about people who were here, you know, 200 years ago," says the actor.
The history of the place appeals to him. So does the fact that his home near Santa Fe is 1,000 miles or so away from Hollywood. In New Mexico, between movies, he can live his life in private.
Lenihan, one of the world's experts on underwater archeology, is no stranger to CBS News. In 1991, during commemorations marking the 50th anniversary of the attack on Pearl Harbor, he guided the late Charles Kuralt through the wreck of the USS Arizona.
Hackman suggested a pirate story, and Lenihan challenged him to start writing first. "So I went home and wrote this chapter that is still in the book," recalls the actor, "strangely enough."
Even though they live in the same town, they worked separately, at home, with all the usual distractions. Hackman wrote his chapters longhand in spiral notebooks.
Unorthodox to say the least, their method was a little like the building of the transcontinental railroad. Picking different starting points, they met in the middle - the middle, in this case, being the Cloudcliff Cafe.
"I would have some pages. He would have some pages. We would trade," explains Hackman. "We'd read 'em over while we were ordering and eating and after a couple of hours, we would have critiqued each other's work and decide where we were gonna go from there."
Hackman and Lenihan, the two diving buddies, couldn't resist writing diving into the book - diving as their characters would have improvised it 200 years ago.
"They take a barrel," explains Lenihan. "They invert it, put weights on it. It's hard to sink a barrel. And they get it down to a depth where the air compresses up into a pocket and have themselves, essentially, a diving bell."
The two writers eventually realized that they had similar early backgrounds. Hackman quit school at 16 and joined the Marines at the end of World War II. Lenihan also left home at 16, to go to college, where he became involved in the civil rights movement.
As for Hackman, even when he was a child in Danville, Ill., he wanted to be an actor. He has made about 70 movies and has won two Academy Awards. Among the characters he created for the big screen are Popeye Doyle in The French Connection and Harry Caul in The Conversation.
"One could say, you know, that I've achieved some manner of success," Hackman says. "But it's like the difference between being a doctor and a lawyer, and being, maybe, a laborer. I feel that, in some ways, that my efforts of being an actor are so all-instinctive, and that I haven't really paid my dues in terms of my contribution to the firmament."
Writing seemed to be a way to accomplish that. But Hackman was surprised at how exposed it made him feel. Wake of the Perdido Star is selling well, but the book has received mixed reviews.
"The fact that you're being judged on your intelligence, and your skill as a writer, and your skill as a storyteller was very tense for me," he says. "And being criticized and finding that you're vulnerable to the critics in a way that I haven't experienced before."
Yet Hackman and Lenihan are considering another collaboration. At 67, Hackman shows no signs of slowing down, even though he has been telling interviewers for yearthat he's planning to do less acting. Among the three films he worked on last year is a soon-to-be-released thriller, Under Suspicion, which also stars Morgan Freeman.
In addition to acting, diving, and writing books, Hackman is a pilot and has raced cars.
Like the roles he chooses, his life is varied and deliberately challenging.
"I need diversity," Hackman says. "I couldn't - I don't think I could stay here (in New Mexico) all the time."