For Johnny Galecki, also known as physicist Leonard Hofstadter on “The Big Bang Theory,” the end of an era is in sight; CBS recently announced that after a dozen years, season 12 will be the last for the 10-time Emmy-winning show, which has spurred household catchphrases and the TV spinoff “Young Sheldon.” Along with Jim Parsons, Kaley Cuoco, Simon Helberg and Kunal Nayyar, Galecki helped the nerd comedy climb to the top of the ratings.

Galecki drew his first mention in Variety in 1987 when he was cast in the NBC pilot “Kowalski Loves Ya,” later named “Time Out for Dad,” starring football legend Dick Butkus. Since then, he has scored a Golden Globe nomination, as well as Emmy and Critics Choice nominations for his work on “The Big Bang Theory,” and rumors are swirling as to whether he’ll reprise his role as David Healy in the upcoming “Roseanne” spinoff, “The Conners.”

When did you first know you wanted to be an actor?

I started talking about it when I was 3, and I have no idea how the word “actor” was even in my vocabulary. No one in my immediate family was an actor, or even in theater or television or anything. We barely had money to go to the movies, let alone buy a theater seat, so it wasn’t part of my family’s lifestyle, and it was just my answer when people asked, “What do you want to be when you grow up?” I really have no idea how I came upon that, but I feel incredibly blessed that that was the case because I see people, even in their 30s and 40s, struggling to learn what their calling is, and I think I just knew it as soon as I could speak.

What was it like having Dick Butkus as your TV dad for one of your first projects?

Of course, being from Chicago, working with Dick Butkus was a big deal. I’ve never been much of a sports fan, but those were big bragging rights for my dad at work. [Co-star] Harriet Nelson — even though her body of work was before my time when I was 11 — she had an air of legend around her. She had a grace about her. She was definitely the most seasoned person. And I vividly remember Beah Richards. She was a wonderful actress. She seemed to take a shine to me very much like an equal on the set, even though there were 70 years between us, and I still think of her whenever we have young kids on a set. I hope I’m just as kind to them as she was me.

How does having that child-actor background inform how you interact with kids on set now?

Some of the best acting lessons I’ve ever received as an adult have been from watching kids work — whether that’s on screen or on set. There are certain walls, certain insecurities, that haven’t been accumulated yet, and there seems to be a real fearlessness and earnestness there that’s rare. You try to retain it as you get older, and it becomes more difficult. People ask me all the time, “How did you get started when you were so young? That must have been so terrifying.” It’s not terrifying when you’re 7. It’s terrifying when you’re 27. The stakes aren’t that high when you’re 7 years old. You can always go back to Little League.

What comedy experience did you gain working with actors like Chevy Chase in “National Lampoon’s Christmas Vacation”?

They don’t depend on many young people to do the heavy lifting of comedy. That was a real challenge because the Rusty role was kind of iconic at that point and did have some heavy lifting comedically, and my timing wasn’t on-point. Chevy would help me out, especially with the timing, and tell me some ad-libs to say. He was very patient and giving of his time. This was only my second time in Los Angeles, and I was 13 on that movie, and he would take his lunch hours and — without telling any of the production assistants or anyone — we’d just get in his car and disappear and go to film lots and visit the sets of “Ghostbusters 2” and “Harlem Nights.” Here I am at 13 being introduced to Redd Foxx and Eddie Murphy and Dan Aykroyd and Bill Murray and Richard Pryor and all these incredible, incredible comedic icons. He didn’t have to do that, and it’s still very touching to me.

Is there an early mistake you made that shaped you as an actor?

I do remember being on the “Christmas Vacation” set, and I was standing there with Chevy and one of the producers and [screenwriter] John Hughes. He was visiting the set, which I think he rarely did. And Chevy said, “In an early draft there was a scene just between father and son. We’ve always had those in the previous two movies, and maybe that scene should come back. Give me and Johnny a scene — just the two of us — to do together.” And John Hughes looked at me and said, “What do you think?” And, 13, not thinking at all, I said, “Well, if somebody thought it was unnecessary enough to be cut from a draft at some point, then I can’t see it ending up in the movie if we did shoot it.” And Chevy looked at me like, “You just talked yourself out of a scene that John Hughes was going to write for you.” That was a big lesson. Just from the look on Chevy’s face, I realized I messed up and that I had just ruined a pretty big opportunity for myself by being a little too honest.