'Castle' season three sneak preview

Exclu: Ready for a first look at the third season of ABC's "Castle"? Below is the opening scene of the season premiere that was just screened at Comic-Con on Sunday. This scene is the first moment that Castle and Beckett have seen each other since the S2 finale:

And below, THR's Philiana Ng details what happened during the "Castle" panel:

Neil Patrick Harris, you have been warned.

The "Castle" crew challenged the "How I Met Your Mother" (and "Dr. Horrible") star to appear on their show, though creator Andrew Marlowe thought Harris was too scared to tackle the crime dramedy.

Star Nathan Fillion had this to say: "I think it'd be really cool to kill him, but ...."

Marlowe hinted at several storylines for Season 3, which premieres Sept. 20 at 10, and takes place months after the second season finale.

"We feel that it's time to challenge Castle and Beckett with someone who is as smart as they are. There is a real feeling of gamesmanship between the two," he said about a possible multi-episode arc spotlighting a serial killer. "It's a character that pops for the audience and [I hope to bring back]."

Marlowe shared that Beckett would be "dealing with [her] mother's murder" throughout the season. "It's an issue of identity for the characters so we don't want to wrap it up quickly," he revealed.

Fillion had a lightbulb moment when he was asked if we would see Castle and Beckett as kids. There could be a "flashback where they met once at kids but don't remember, pulling some pigtails on a playground. 'Kate, stop pulling my pigtails!,' " Fillion joked.

As for Castle's family life, we will see his daughter acting her age.

"We will see Alexis dating. I see a little anxiety coming [for Castle]," executive producer Laurie Zaks shared. Marlowe added: "He's always pushing his dauther to take chances that he did, but when she does, he doesn't really like it."

During the fan Q&A portion, Fillion gave double rainbow makers, his personal party wristbands, a pack of Oreos and a nail file to the lucky interrogators. And proving his online presence, Fillion tweeted out responses to two phrases (both "Double Rainbow"-related) for the audience to yell out should he use them during the panel (which he did, multiple times).

A highlight was Fillion and co-star Stana Katic reading from "Heat Wave," the novel Fillion's character Castle wrote on the show. In the series, the book is being turned into a movie ("In this city, when temperatures rise, so does crime. Everything. Heats. Up."). The sequel "Naked Heat" is out in stores Sept. 28.