Jason Ritter knows how to make the most of an opportunity. When the 32-year-old actor was invited to guest star on the first season of the NBC dramedy Parenthood, he was only supposed to appear in a three-episode arc as sexy, mild-mannered high school teacher Mark Cyr. Cut to Season 4: Ritter has an Emmy nomination under his belt, his character is engaged to fortysomething single mom Sarah (Lauren Graham) — age difference be damned — and he's even managed to wangle his way into the official family portrait. But all that could be at risk, thanks to the recent arrival of Sarah's new boss, Hank, played with crotchety magnetism by Ray Romano. Ritter opens up about his status as an almost-Braverman, life lessons from his late dad, John, and what's really beneath the nice-guy façade. (Spoiler alert: Unless he's really good at hiding it, there's no secret dungeon filled with puppies.)

TV Guide Magazine: Guest stars on Parenthood come and go. What's the trick to your staying power?

Ritter: It was sort of a slow burn. I loved those first three episodes I did so much. Then they kept bringing me back — even while I was doing my [2010-2011] show The Event. I'd read every script thinking, "This could be it!" But now Mark feels like a real part of the show. At least for now. I got in the [family] picture!

TV Guide Magazine: Is your chemistry with Lauren Graham as good as it looks?

Ritter: I know there are actors who can pretend they're in love but in real life cannot stand the person. For me, it really helps when there's a friendship. It's so much fun to come into work and play around with Lauren and flirt and try to make her laugh and then go home. That is such a great day.

TV Guide Magazine: So is Hank actually going to mess up all that Mark-Sarah magic?

Ritter: That's the question. The biggest threat is always the idea that you would be happier elsewhere. She and Mark have been through a lot, but now that they're living together, the realities are different from the fantasy dating section of the relationship, when no one's ever leaving the seat up. Plus, Hank is so real and grounded, even if he does have a bleak outlook. Mark has a lot of ideals.

TV Guide Magazine: If Sarah cheats on Mark, will fans riot?

Ritter: Maybe fans would riot. Here's the thing I love about the show so much: It's about life's little triumphs and setbacks, idiosyncrasies of the human heart — so you don't need to have them have a full-on affair to break Mark's heart. There are lots of threats that don't necessarily end with Hank and Sarah in a hotel room. But I don't think Sarah would do that to Mark. Though the sooner Hank gets out of town, the better.

TV Guide Magazine: On the November 20 episode, Mark's dad skills will be put to the test when he walks in on Sarah's teenage son, Drew (Miles Heizer), in a compromising position.

Ritter: Mark's not a disciplinarian. He's finding himself in all of these positions he doesn't know how to navigate but wants to so badly. Like he thinks, "I should give Drew the birds-and-the-bees talk," which of course Drew has heard before.

TV Guide Magazine: Did having TV icon John Ritter as your father make it harder or easier to break into this business?

Ritter: When I was starting out, I would go in for auditions and I would get overwhelmed by my own insecurities. I would have my little jokes prepared, and the casting director would say, "Before you start, I want to tell you that your dad was so funny" — and give him a nice compliment — and I would think, "Oh, my God, you're about to see how far I am from him." I wasn't just a guy in the room; I was humiliating my family. But there were ways it helped a lot, too. I was going into a business where he had been really gracious to a lot of the people around him.

TV Guide Magazine: He passed away in 2003, two weeks before the premiere of your first series, Joan of Arcadia. Did he ever get to see you act?

Ritter: He saw the pilot of the show. I did a play that both he and my mom came to see, and he saw me in episodes of Law & Order. He'd tell me he was proud of me. I do wish he'd been able to see all of the things since his death, but I also think somewhere maybe he is seeing them and all the things in the future. My dad always had an eye on the ball as far as what was important; this whole idea of celebrity really bugged him. He told me I have total control over whether I'm an actor or not — I can go into the park and perform something, and that makes me an actor.

TV Guide Magazine: You've played a lot of good guys. Be honest: Are you ever a big jerk in real life?

Ritter: I can be very frustrating! I'll be at dinner with friends and if someone comes up to me, rather than spending a couple minutes saying hello, I engage with this person and miss out on my evening instead. I don't understand why. The conversation with the stranger will even wind down, and I'll go like, "So, what else?" It's really weird. Even though it's not directly being a jerk, it is still jerky behavior.

Parenthood airs Tuesdays at 10/9c on NBC.

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