While Parsons admitted to having "suspicion in my heart" that his two-year contract for seasons 11 and 12 of the sitcom would be his last, that idea didn't solidify until the penultimate season taped its final episode.
"That summer I went to New York to do Boys in the Band on Broadway and I think anything I felt got affirmed," he said. "It was a very intense summer. Partly because of the part and the experience of doing the play, but more because [I was so busy]."
One day after wrapping season 11 of Big Bang, Parsons flew to New York for Broadway rehearsals and, not even a week later, was in previews of Boys in the Band. Though his day off fell on Sundays, he didn't get to rest that first week.
"On that Sunday that I had my first day off, I shot a commercial," he recalled. "I had a contract with Intel and so I scheduled that. I was exhausted."
On top of his exhaustion from working, Parsons was "more than anything" upset about his and husband Todd Spiewak's 14-year-old dog's declining health.
"I'll never forget that walk around the park to let him go to the bathroom before we went to the commercial, he just looked so bad and I was so tired and Todd was like, 'We've got to go. We've scheduled this. They fit everything around your schedule.' And I just started crying," Parsons admitted through tears. "It makes me upset now. I was like, 'This dog's gonna die while I'm off working. I feel so bad.'"
"So I went and did the commercial, I came back, and then Monday I went to do the play and he had a really bad seizure that night, so I knew that we had to make a decision... We called somebody and the person comes to put the dog to sleep at home," he continued. "So that happened Tuesday. It really upset me. Still does."
After his dog's death, Parson had six Broadway performances to get through until his next day off, but that proved to be a challenge.
"I was in the Saturday matinee and I kept thinking, 'I don't know how I'm going to get to the end of this performance.' I was just so beaten down," he said. "But I did."
While walking out for curtain call at Saturday's matinee performance, though, Parsons slipped and broke his foot.
"It was the scariest moment for the next couple of days because... I felt like I was at the edge of a cliff and I was teetering and I saw something really dark below, between the death of the dog and [the fact that] I don't know what they would've done if I couldn't have gotten back on for the play," he said while getting choked up.
All of those things combined to form "a really intense" summer for the actor.
"I had this moment of clarity -- that I think you're very fortunate to get in a lot of ways -- of going, 'Don't keep speeding by... Use this time to take a look around,'" he recalled. "And I did. I was like, 'I gotta make a move.'"
On top of all of that, Parsons was forced to face the fleeting nature of life as he thought about his late dad.
"My dad had passed years before, but he was 52. And I realized that at the end of season 12 I would be 46," he said. "... It was just a context thing."
When he got back to L.A. to film what would become the final season of The Big Bang Theory, Parsons explained the reasons behind his decision to creator Chuck Lorre and writer Steve Molaro.
"I said, 'If you told me that like my father I had six years left to live, I think there's other things I need to try and do. I don't know what they are, but I can tell that I need to try,'" he said. "... It was kind of clarity thrust upon you, as Shakespeare might have said... I was like, 'OK. Let's take charge here.' And so that's exactly what happened."