[Warning: The following contains major spoilers from Thursday's Season 11 finale of Bones. Read at your own risk]

Bones'Season 11 finale ended with a major cliffhanger and a shocking return.

Zack Addy (Eric Millegan) revealed himself to be Brennan's (Emily Deschanel) kidnapper in the final moments of the episode. Zack, a former member of the Smithsonian team, hasn't been seen since the early seasons of the show when he was framed for the murder of a lobbyist and sent away to a mental hospital.

The implication of this finale is that Zack at least has ties to the puppeteer serial killer that has been stalking the team for the last half of the season. Now he has Brennan, and the team — including Booth (David Boreanaz) — has no idea where Zack has taken her, or why. TVGuide.com talked with Millegan about his return to the show and what Zack has planned for Brennan going into Bones' final season.

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When did they ask you to return to the show?
Eric Millegan: In January, I got an e-mail from my agent saying they wanted me for the last two episodes of Season 11. This was before they knew they were going to have a twelfth season. I think they were going to bring me back and resolve my character in two episodes...I kind of had to wait for a while. They told me to keep it a secret, "If it gets out we're not doing it." I was told it was probably going to be in April...Eventually, I got the offer and the script. It was exciting because I found out that not only was I coming back but I was the cliffhanger for the season, which set me up nicely for next season.

How has Zack changed since the last time we saw him?
Millegan: He has done a lot. I say at the end of the episode, "We have so much to talk about." Obviously, we don't know what that is and the audience won't find out until January. I start shooting on Monday and he's been doing a lot. I wish I could say, but he's been through a lot. He's done a lot. It's amazing how they've developed my character from the beginning of Season 1. I can tell you, you're going to love it and people will not be disappointed.

What is it about Brennan that Zack is so specifically drawn to?
Millegan: I won't say he loves her because I don't know if he knows specifically what that means. He cares about her very much because they have a meeting of the mind. They understand each other's logic. She's a forensic anthropologist. In Season 2, he became a forensic anthropologist. There's a lot of respect between the two of them. He can relate to her. She's very, very smart but she's always said throughout the show, "I don't know what that means." She doesn't always respond to things in the way that other people do. Zack loves that I think.

What is Zack's ultimate goal now that he has Brennan?
Millegan: The last time we saw Zack in real time was 8 years ago. There's a big hole with what's going on with him. He respects her. He is innocent of murder. He does feel that he was the accomplice to a serial killer so he feels like he would go to jail if people found out what he did. He wants to stay at the mental hospital where he feels he would do better. He is not crazy. Sweets knew he wasn't crazy. Now, we have to deal with the fact that two years ago Sweets was murdered so nobody knows that Zack didn't kill anyone. In episode 1 of Season 12, everyone is going to find out exactly what Zack has been up to. It's a big episode. You find out everything.

Wait, so he's not actually the puppeteer?
Millegan: I can say this: when they announced that I was going to be on and they were doing this puppeteer storyline, I talked to Tamara [Taylor]. She said that when she heard that I was coming back, she and TJ [Thyne] went to Michael Peterson, who wrote the finale and [next season's] opener, and said, "You can't make him a killer. We've been through that already. You have to make him the hero." I have my colleague going to bat for my character.

Does he have any unfinished business with the rest of the team?
Millegan: Yes...He cares. He cares about people. He had a close relationship with Hodgins, with Brennan and even with Angela and Cam. He had his own special relationship with Booth, which is explored in [the Season 12 premiere].

Bones Season 12 will return in 2017 on Fox.