On April 5 and 6, HBO will premiere Oscar- and Emmy-winning documentary filmmaker Alex Gibney's Sinatra: All or Nothing at All, a two-part, four-hour documentary about Frank Sinatra told through the set list of his 1971 "retirement" concert in Los Angeles.

To talk about the film and Sinatra's career and life, Billboard brought together Gibney and James Kaplan, who, on Oct. 27, wlll publish the second volume of his definitive biography of the singer, Sinatra: The Chairman. (Kaplan published the first volume, Frank: The Voice, in 2010 and has spent the last 10 years researching Sinatra's life.)

TV Review: HBO's Frank Sinatra Documentary 'All or Nothing at All'

Gibney and Kaplan spent almost an hour discussing the treasure trove of previously unheard recordings that Gibney features in the documentary, including a revealing and candid first-ever interview with Sinatra's first wife Nancy.

The filmmaker explains that despite getting "fantastic" cooperation and access from Sinatra's family, they did not agree with his decision to end his story in 1971, because Sinatra returned to the stage just a few years later and continued to perform into the 1990s. "The Sinatra family was extremely resistant ... to this idea that '71 would kind of be the end, more or less."

But Kaplan agrees that Sinatra's life "changes in a lot of different ways" after his comeback in 1973. "He has essentially stopped making movies. He makes very few records after 1973. He becomes completely a concert artist playing very large venues," Kaplan explains. "And, what's more, the not-so-secret history of Frank Sinatra, of his life with many, many, many women comes to an end," after he marries his fourth wife, Barbara Sinatra, in 1976. "So, here's this monogamous guy who's going around playing concerts. It's just not the same kind of story." 

Kaplan also recounts the behind-the-scenes drama that took place during CBS News anchor Walter Cronkite's 1965 interview with Sinatra, footage of which is featured in All or Nothing at All. "Cronkite was under acute pressure," Kaplan says. "Sinatra pulled the plug on the interview in the middle of that interview. He took Don Hewitt, Cronkite's producer, outside -- they were at Sinatra's Palm Springs home -- and completely reamed him out. He said, 'This interview is at an end' -- simply because Don Hewitt had asked Walter Cronkite to ask bout the maffia." He adds: "Somehow they were able to talk Sinatra off the ledge and get him back, but what resulted was very anodyne." 

Check out their engrossing conversation here: