NCIS

On Tuesday's holiday episode of NCIS, Gibbs (Mark Harmon) will get a special visit from his father (guest star Ralph Waite), and the family connections won't stop there.

Check out photos of the NCIS cast

Waite, 81, first visited the CBS hit procedural in Season 6. This time around, he will exhibit some unusual behavior that leaves Gibbs perplexed as he and the team investigate the possible hate-crime killing of the son of a retired colonel turned preacher.

Even though the episode has Christmas themes, Rocky Carroll warns that it's not exactly a heartwarming hour. "Gibbs is still coming to terms with his dad," Carroll tells TVGuide.com. "We'll get a little more sense of their relationship, but we'll never get warm and fuzzy because that's not who Gibbs is. But the fans will get what they want."

Carroll says fans will continue to see more of the family lives of their favorite characters in the second half of the season. Robert Wagner, for example, will join the cast in the series' 150th episode on Jan. 12 as the father of Tony DiNozzo (Michael Weatherly).

NCIS: Robert Wagner to play Tony's dad

"There's been a real emphasis and a focus towards family and family ties for all the characters," Carroll says. "We're going to see DiNozzo's family, and after 7 seasons, it sort of warrants that. You get to know somebody for a while, [and] you want to meet their family. I think in a sense, this season we're taking our audience home to meet the folks."

But the family focus won't completely rewrite the show's DNA. "The star of NCIS is the agency," Carroll says. "It is the thing that makes [the show] work. That has always been the mantra, and it will never get too far away from the actual operations. Family is something we're going to touch, but ... there's always going to be a death investigation and a bad guy to bring to justice. We just like these characters so much that we want to see the other people on the periphery of their lives."

But the operations could always lead to more family stories, particularly with Ziva (Cote de Pablo), whose father is the head of the Israeli intelligence agency Mossad. "Ziva is caught in the middle between her family relationships and her extended family at NCIS," Carroll says. "There's still going to be some international tension between the NCIS and Mossad. That's still going to be addressed, and I think it's the tip of the iceberg."

Read Rocky Carroll's NCIS and NCIS: Los Angeles blog

The irony, however, is that Carroll's character, Director Leon Vance, is a family man whose wife and kids have been featured in the show. Carroll hints that instead of exploring his relationships at home, the writers are looking for opportunities to reveal more of Vance's back story.

"[Vance is] sort of a wildcard," Carroll says. "It's not essential to the success of NCIS that Vance's character be an open book. The air of mystery is the thing that makes the dynamic work between Vance and Gibbs. We're going to keep trying to have that going as long as we possibly can. Audiences are slightly off-balance with this character, and that's what we're going for."

NCIS airs its final episode of 2009 on Tuesday at 8/7c. New episodes return on Tuesday, Jan. 5.