The filmmakers behind John McCain: For Whom the Bell Tolls, tell PEOPLE the Arizona Republican battling brain cancer is "a down-to-earth, decent guy"

In the sunset of John McCain’s 60-year service to the nation — first in the Navy and then in the United States Senate —filmmakers Peter, Teddy and George Kunhardt were granted extraordinary access to the senator and his family last summer as he started down the hardest road of his life so far: battling stage 4 brain cancer.

“He is the Republican lion of the senate,” Teddy Kunhardt tells PEOPLE before his almost two-hour documentary, John McCain: For Whom the Bell Tolls, debuts May 28 on HBO.

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Says Peter: “We saw that he’s very different from most politicians in that he is a down-to-earth, decent guy.”

Image zoom Jim Lo Scalzo/Epa/REX/Shutterstock

McCain’s only pre-conditions: He told the filmmakers, “‘I don’t want you to shy away from my failures, from my screw ups,'” recalls Teddy, adding that the Arizona Republican added, “‘I want the American people, and specifically my colleagues, to know we’ve got to start working together.'”

The filmmakers found everyone from Barack Obama to George W. Bush to longtime friend Joe Biden extremely eager to make time to pay tribute to McCain on camera. And while much has been made about the deep bonds between McCain, Lindsey Graham and Joe Lieberman, filmmakers were surprised to discover a real warmth between McCain and 2016 democratic presidential nominee Hillary Clinton.

“She genuinely loves this guy and thinks the world of him,” says George Kunhardt. “They both lit up when we discussed them with each other. They both genuinely love each other. It was a very emotional and touching thing from our point of view.”

This was far from the only surprise in store for viewers of the film, which includes scenes of a relaxed and reflective McCain at his home in Sedona last summer — soon after his July diagnosis with glioblastoma, one of the deadliest forms of brain cancer.

Image zoom Jacquelyn Martin/AP

In an interview on his porch, McCain revealed a shocker regarding his choice of Sarah Palin as his running mate for his 2008 presidential run.



“I should have said: ‘Look, Joe Lieberman is my best friend, we should take him,’ ” McCain says in the film. “But I was persuaded by my political advisers it would be harmful, and that was another mistake that I made.”

Image zoom Kiichiro Sato/AP

McCain’s honesty also extends to moving reflections on his mortality: “I love life, and I want to stay around forever. But I also feel that there’s a great honor that you can die with.”

During their off-camera time in Sedona, the Kunhardts saw McCain “constantly talking to people visiting him, or on the phone,” says Teddy. “He does Twitter stuff.”

McCain also enjoyed birding and reading, with the filmmakers often finding him engrossed in a book. “We asked him about that and he said, ‘My whole life I’ve been fascinated by books,'” says Teddy. (The film’s subtitle comes from McCain’s favorite book, the novel by Ernest Hemingway.)

Teddy tells PEOPLE the idea for the film came immediately after he read a July New York Times story breaking the news that McCain was diagnosed with glioblastoma.

RELATED VIDEO: John McCain Has Been Diagnosed With Brain Cancer

He immediately pitched the idea to George and Peter, and together they pitched the film to HBO later that day. “Within seconds they said, ‘Let’s do it,’ and we hit the ground running from there,” says Teddy.

In addition to filming McCain in Sedona, they followed him to Washington, and over time discovered why someone like Hillary Clinton would have genuine love for the 81-year-old.

The filmmakers also flew to Vietnam to highlight the more than five years McCain spent as a prisoner of war, and where he was tortured and spent time in solitary. In addition, the film documents the dissolution of his first marriage and the rise of his political career through archival footage and interviews with his children, his ex-wife, Carol, and his current wife, Cindy McCain.