Looks like G. Callen isn't such a lone wolf after all.

On Sunday's episode of NCIS: Los Angeles, Callen (Chris O'Donnell) finally started to get some of the answers he's been looking for his entire life, when his father Garrison (Daniel J. Travanti) makes an unexpected appearance in Los Angeles when a woman is poisoned.

As it turns out, Callen has a few blood relatives after all. At the end of the episode, Garrison informs him that he was romantically involved with the woman who was poisoned, and her daughter is Callen's half-sister. She has a child as well, meaning Callen is an uncle. The news hits Callen like a lightning bolt, and the episode ends with him telling Garrison that he's refusing to hold on to any more family secrets.

TVGuide.com chatted with O'Donnell about his reaction to Callen's discovery, and what kind of a relationship he and Garrison will have going forward.

TVGuide.com: Callen's been searching for answers his whole life, and now he's in a position to actually get them. What is his reaction upon unexpectedly seeing his father in the hospital?
Chris O'Donnell:
It's been eating at him his entire life, and ... he's learned to live with that. Then, all of a sudden, this triggers it. He's got a job to do, but he's got a lot of questions to ask on his own part, and that's a real struggle for him. I think the unknown is always scary. And I think that he's probably come up with a thousand scenarios in his own head of reasons and explanations. All of a sudden when you think something that's maybe not ever going to be attainable is suddenly attainable, you've got all sorts of emotions swirling around. There's fear. There's anxiety. There's anger. There's a lot of things going on. And I think it makes someone like Callen... we don't see him that vulnerable very often. This is something that probably cuts to him more than anything. This is a real trigger for him.

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What will Callen and Garrison's relationship look like going forward?
O'Donnell: I know we're going to see more of Garrison this season. We haven't really gotten into it yet. I know that for part of the time, he is still hanging around Los Angeles, and so there is interaction [between them], but we haven't explored that completely. But yeah, here's a guy who, his family is effectively Sam Hanna's and the group he works with at work, and suddenly he has actual family, like blood family - well, partial blood family - that he didn't know about. And I think there's going to be a lot of opportunities there for him. I think it will affect him in a lot of ways. Maybe this will take him out of his shell a little bit.

Callen says at the end of the episode that he won't forgive Garrison. But do you think, after their conversation, that he understands him a little more?
I think it gives him some insight. I think he starts to understand things, but forgiveness is something that takes time. Hopefully there is time for him to forgive him, and then time to heal things. But I think that part of the explanation that Garrison gives him, on certain levels Callen can relate to that, knowing some of the choices he had to make along the way and people that he may have hurt along the way, for reasons he could never explain to people. That's part of the lives that these guys live. When they go under or they're living undercover as federal agents, there's just things that they can't tell people.

Does Callen trust his father? Should he?
I don't know that he fully trusts him. I think he's getting there, and they take some big steps in this episode, but I think they still have some work to do.

When Callen goes into the house to save his father's life, what's going through his head in that moment? Is he looking at him as a father, or just as someone who needs to be saved?
For me, it was definitely that this was not just an average case. The stakes were raised. This is my link to my past. There are answers that I need. This is my one link, and I can't lose that. So, aside from doing our job and wanting to save someone that's being tortured, my God. The stakes couldn't be higher for him.

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What about in the interrogation scene? Did you play it as if Callen was actually able to set his personal ties aside?
O'Donnell: One of the exciting parts of playing Callen is to imagine playing somebody that grew up not knowing anything about his family, his father. I come from a big family. I was so close with my father before he passed away, and I think just trying to put yourself in his shoes, and then imagine what his life has been and what it could have been, and the fact that, here is this guy sitting here and you have a job to do. There's just a lot of things that come into play, and you kind of play with it. You see how you react. Daniel and I read the scenes together. There's also the way you interact with your fellow actor and the way he's playing it. You're playing ball.

Does Callen struggle at all with reconciling what he knows about his father with how he perceives himself?
He'll certainly gain some insight about himself, just spending time with his father. I think that as you get older in general [with] your parents, you've been away from them a little bit, and you suddenly watch them and you hear them and you start thinking, oh my God, I really am like my mom, or I really am like my dad. I think when there's space or when you're away from your family for a while, you come back and you kind of see it. Sometimes when you're together all the time when you're growing up, you don't really acknowledge it because you can't see it.

Is there anything you can tease about the mole investigation?
It's a multi-episode situation. I just read two of them, and it's really well-done. The script for the second one may be one of the best scripts we've had. It's really clever the way they did it, and it's not fully resolved yet, but we are starting to get a peek at who the mole may actually be. And I think it'll be shocking for the fans. ... There's some twists and turns that you really did not see coming. Having been around for 180 shows, I usually know what to expect, and this one was pretty clever, I have to say.

NCIS: LA airs Sundays at 8/7c on CBS.

(Full disclosure: TVGuide.com is owned by CBS.)