The search for Serena’s father is officially over.

After being MIA for nearly three seasons, ‘Gossip Girl”s William van der Woodsen (played by William Baldwin) makes a surprise visit on the Upper East Side tonight, where he’s reunited with Serena, Eric and — wait for it — Lily’s current husband, Rufus.

How does his long-awaited reunion with Serena go? What trouble might he have in store for Rufus? And what happens when Eric comes out of the closet?

In an exclusive interview, Baldwin spoke to TV Squad to give us the rundown on his three-episode arc, which begins tonight at 9PM ET on the CW.

How did you initially become in involved with the show?
Last season, I was offered something on it and [they] basically said, “Come on and do a guest spot for an episode.” And I thought, if I’m gonna do this show, especially coming off ‘Dirty, Sexy, Money,’ I don’t want to go all the way to New York and just work for two days on one episode. I wanted them to find something juicy and racy, and find a little arc for me and get three, four episodes. So I hinted about it. I said, “I really like the show, I’d love to be a part of it. Can we find something more substantive or more significant?” And I respectfully passed on that one.

But I was intrigued by [the show], because they sent me a couple of episodes — I’ve got three little kids, I’m not hitting the movies and watching a lot of TV, so I had not seen a lot of the show — but I was well-aware of what the show was and who was on it. I knew some of the characters’ names and I knew some of the storylines … and certainly the kids who star on the show. They sent me a couple of episodes — I had seen one before but I hadn’t seen one in a year or so — and I sat in bed with my computer with my wife and we watched one and, I mean, you couldn’t stop watching it.

Serena has been searching for William for almost three seasons now. How does the big reunion go?
It’s interesting, because you imagine you’re in that situation where you’ve been looking for your father — you’ve had minimal communication with him over the recent years and you feel abandoned and rejected, and once again that’s caused all sorts of insecurity and inadequacy … So she’s sitting here wanting her father and actively seeking her father, but then she’s finally reunited with him, and she’s in the same room as him, and she wants to touch him, and she wants to hug him and she wants to tell him that she loves him and she wants to start making up for lost time … And then, of course, you get this see-saw of emotions. But what starts to creep back in? The doubt and the fear and the resentment and the anger … So she really is guarded.

How will William’s return to New York affect Lily’s relationship with Rufus?
That’s the interesting balancing act, because he comes back as the biological father [of Serena and Eric] and it’s a very complex issue, a whole menagerie of issues, psychologically and emotionally, that sort of come into play. He has the right to have a relationship with his family, but at the same time, he’s gotta do this balancing act of respecting the boundaries of what Lily has now established in her relationship with Rufus. Of course, there’s the whole past with Rufus — all the tension, and insecurity and inadequacies that have been created over the years between William and Rufus. I think that in his heart and in his mind, he’s coming back to make things better and to make things right and to re-establish the love, and the trust and the respect first and foremost with his children; and, of course, with Lily … Having said that, that all sounds good in theory. In practice, it doesn’t necessarily work out that way.

Does William get to do any father-son bonding with Eric?
That relationship is more of a struggle and is more challenging for Eric. He’s more resistant. He has more of a chip on his shoulder … He’s less trusting than Serena, he’s not like, “Gee, swell. Dad’s back. Can’t wait to have him back in my life.” He’s like, “You’ve been gone all these years. You left when I was a kid. You abandoned me.” Kind of [like], “Go f*** yourself.” He needs some more [time] before he’s going to start to warm to the possibility of me back in his life again.

Does Eric tell William he’s gay?
It’s dealt with in a really great way — the way it was written and the way [Connor Paulo] performed it. I remember the exact scene. I’m trying to break through and have a conversation, and he’s just not having it. And I said, “You know, there’s this benefit. I’m being honored and I’d like you to come, and perhaps you want to call one of your girlfriends and ask her to come.’ And he’s like, “Two things: A.) That’s never going to happen, B) because I’m gay.” I don’t remember the exact wording but his performance in it was really great. I think he says something like, “If I did, [I'd] take my boyfriend, because I’m gay, by the way. I don’t even want to go.” It’s a really good scene. He kind of just slams the door on my face and says, “If you knew anything about my life, you would have known that this is who I am … You haven’t been around, but if you had, you would have known by now that I’m gay.”

You’re currently signed on for three episodes. But since Serena’s father is such an integral part of the show, do you think you’ll be back for more?
The way it was written sort of lends itself to that. There’s a fun, juicy cliffhanger that they set up for the end of my arc. It’s sort of in the eye of the beholder. I have no commitments from anybody, but I think the way it’s written and the way the arc played itself out and the culmination with the cliffhanger, it sort of leaves it open-ended. I think you’ll have to see reactions … But you gotta remember this is a guy who cut and run 15 years ago, so sometimes you don’t know which William van der Woodsen you’re going to get. That’s the beauty of William. He’s not a bad guy who has the authority to do good and nice things. He’s a good guy … Good people have their Achilles [heels], their flaws and their weaknesses, and this is a good man who has made mistakes and is trying to make up for it and make it right.

In the end, I think the long-winded answer I just gave you is, I would be surprised if he wasn’t back. But there’s certainly no commitment … I think when you see the way the arc plays out and you see the way in which the storyline resolves itself, the future of William van der Woodsen’s involvement with his family is not clear and it leaves it open for debate. If I were a betting man, I would bet that it would be a lot more fun to have William van der Woodsen come back with a juicy, delicious, decadent, deplorable, stirring-the-pot storyline once again — maybe next season and beyond. I’d come back. I’d do whatever they want to do. If they want to make me a series regular, maybe I’d move back to New York for that.

Your last major TV gig was ‘Dirty, Sexy, Money,’ which was canceled after just two seasons. Do you think ABC gave it a fair shot?
It’s really hard to say, because we were dealing with circumstances that haven’t been dealt with since the Great Depression … They ordered 13 [episodes], then we got orders for the back nine, and then the writers’ strike hit and we never shot the back nine. So they ordered 13 episodes for a second season, which we shot, and in that hiatus during the writer’s strike, they replaced the show runner and they brought another guy in, and he sort of wanted to make some adjustments and take it in another direction with the support of the network.

I’d say, if there wasn’t a global economic meltdown and a writer’s strike, then we would still be on the air … Look at the above-the-line [budget] on this show: You’ve got Blair Underwood, Lucy Liu, Jill Clayburgh, Donald Sutherland, Billy Baldwin — all these people are getting paid. I don’t know what the per-episode cost was on that show, but if I were a betting man, I’d say it was north of $3 million an episode. That’s not cheap. I think they did the best they could do considering the circumstances. If the show had numbers that were a little bit better, or if the show was a little bit cheaper, or certainly if there was no writers’ strike or credit default swaps or derivative whores on Wall Street, we’d all be singing a different tune right now. Wouldn’t that be nice if I was in the middle of season 3 on ‘Dirty, Sexy, Money’ right now? I miss it. We had a lot of fun doing that show.