From the time he was 25 until his death in 2016 at the age of 66, Garry Shandling kept journals — lots of them.

“Garry was working on a project at one point where he was trying to figure out how to use his journals as a way to talk about his journey in comedy,” explains uber writer/producerJudd Apatow. “In the journals he talks about putting a book out called The Zen Diaries of Garry Shandling. I felt like it was something he wanted to share with people.”

That’s when Apatow when took over. In this exclusive teaser of the four-hour HBO documentary that will feature appearances by the likes of Jay Leno, Conan O’Brien, Jim Carrey, Sacha Baron Cohen, Sarah Silverman and Jerry Seinfeld, Apatow reveals how Shandling was an important mentor “who was a mystery to me.” Before he began the project, Apatow made sure to read all of Shandling’s diaries so he could tell a complete story of the comedian who found great success on The Larry Sanders Show but also experienced feelings of loss and betrayal by working in Hollywood.

The doc will covers Shandling’s rise on the stand-up circuit to becoming a TV star, along with the lawsuits over The Larry Sanders Show.

“I feel like he had a neurotic voice in his head and the journals were how he tried to talk to his neurotic voice and calm it down and give it wisdom,” said Apatow, who wrote for Larry Sanders from 1993 to 1998. “In the early years, they were traditional journals. He talked about his struggles at the Comedy Store and what his dating life was like. But then he started to dissect comedy, to figure out how it works. He was an engineer so it became almost scientific in his analysis. Then the journals became more psychological in order to connect with his higher self. There are a lot of Buddhist phrases. He’d write things like ‘you’re missing something by thinking.’ There’s a lot of talk about non-attachments and living in the moment and trying to not judge.”

“The journals are generally not funny,” Apatow continued. “But everyone once and a while, in the middle of a very serious section, he’ll just write a joke. So it will be him worrying about something, and then it will just say, ‘I shave one leg so it always feels like I’m sleeping with a woman.'”

Part 1 of The Zen Diaries of Garry Shandling airs March 26 on HBO. Part 2 airs the next night.