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When you’re the star of a major TV show — like, for example, “The Big Bang Theory,” Chuck Lorre and Bill Prady’s massively popular CBS sitcom that ran for 12 years and the same number of seasons, you probably have to follow some basic rules. An understandable one is that, as an actor, you need to keep yourself physically safe as best as you can, because an injury could be disastrous for the entire production. Apparently, “Big Bang Theory” stars Johnny Galecki and Simon Helberg, who played friends and California Institute of Technology colleagues on the series from start to finish, didn’t think about this when they kept renting ill-advised boats whenever the cast went to Comic-Con together.

As Jessica Radloff revealed in her 2022 book “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series,” Galecki and Helberg, despite knowing very little about boats at all, started a super-dangerous tradition whenever the whole gang found themselves in San Diego for the massive convention. “For some reason, Simon and I had this thing where we have rented a lot of boats together, which makes no sense because neither of us are sailors,” Galecki said. “But every time we went to Comic-Con and we’d rent a boat, he’d buy a captain’s hat, and we’d just endanger everyone’s lives.”

Prady, understandably, had a much different and much less casual memory of this whole thing. “When they told us the story of how they all chartered a boat but none of them knew what they were doing, I just remember thinking, ‘Well, this almost ended very badly,’ and my stomach just flipped.” After saying the only other time in his life that he’d felt that sick and concerned was after his daughter was in a non-fatal but still scary car accident, Prady said, of how he felt upon hearing Galecki’s confession, “So to answer the question, nausea.”

Jim Parsons, who played Sheldon Cooper throughout the entirety of “The Big Bang Theory,” told Jessica Radloff that Chuck Lorre was furious, and it’s easy to see why! “Chuck was also apoplectic about it when he heard. He was like, ‘Wait, what? They did what?! Do you know how much money and insurance is in that one boat?'” 

Despite that memory, Parsons insists that he barely remembers anything about why or how he ended up getting on the boat in the first place. “Now, for what it’s worth, I don’t remember getting on the boat or accepting the invitation [or] plan to be on the boat,” Parsons mused. “It just happened. I remember being on it. But looking at it now, I’m like, ‘I don’t know if I would have said yes to that. I don’t know who that was that said, yes!’ I guess I was like, ‘Well, somebody said we were going to do it, so let’s do it!'” Apparently, there was one saving grace for Parsons: “Thank God I was sober at the time. I don’t even know what body of water we were in. I mean, by the grace of God do I still walk on this earth after that.”

If your instinct is that nothing about Parsons’ alleged attitude sounds at all like the overly cautious Sheldon, fret not, because Johnny Galecki fully blew the whistle on his co-star. “Jim is right,” Galecki said of the conceit in general. “[Helberg] and I trailed off from the group and returned with a boat.”

From there, however, Galecki said Parsons put on a Sheldon-worthy performance. “Jim was very hesitant to come aboard. And absolutely refused to jump off into the water, which I think the rest of us did whenever or wherever Captain Helberg and I deemed our location a safe depth.” Now that’s method acting (although, frankly, Sheldon never would have stepped foot on the boat in the first place).

After reminiscing about his deeply ill-advised boat adventures, Johnny Galecki admitted that it was a terrible idea, and that he now understands why Chuck Lorre and Bill Prady were so freaked out. “We truly didn’t understand the financial investment of billions of dollars at hand until we saw our bosses’ reactions to such behaviors. And also started to notice how the studio would split us up into different cars while on press tours and such — ‘Oh, I see. So if this car rolls over a land mine, only half the cast [and] half their investment is dead. Got it.'”

“And then Kaley’s accident with her horse years later,” Galecki went on, referencing the devastating horseback riding accident that Kaley Cuoco, who played Penny throughout the series, endured that ended up taking her out of two episodes entirely. “That really put into perspective how many people’s livelihoods our actions affected. Before then, we just saw ourselves as a weirdo, ragtag bunch of scrappy actors who didn’t think enough of ourselves to understand we were of value to many people we cared about deeply.” (Cuoco’s accident, to be clear, was extremely serious; after a horse threw the experienced rider, it trampled her leg and she almost needed the limb amputated.)

Lorre retaliated in his own way. On one of his many custom title cards — which you can see on Lorre’s website — he lists some important rules for his “Big Bang Theory” performers. “Following Kaley Cuoco’s horseback riding injury, I’ve instituted new rules governing acceptable leisure activities for the cast of ‘The Big Bang Theory,'” the card begins. The first rule makes sense, with that in mind. “No friggin’ horses,” it declares. “This includes those found on merry-go-rounds and in front of supermarkets.” Apparently, Lorre did not forget the boating incident, based on the fourth rule: “The only permissible boating activity at Comic-Con is in your hotel room bathtub.”

“The Big Bang Theory” is streaming on Max now.