Robert Wagner and Michael Weatherly Robert Wagner and Michael Weatherly

NCIS fans can generally expect a focus on family and Tony DiNozzo's wisecracks, and the 150th episode will feature both. But an investigation of family could lead to a less quippy Tony.

Check out photos of the NCIS cast

"I think there is a little bit of the silliness in the first 150 episodes of Tony," Michael Weatherly, who plays the character, tells "Moving forward, some of the silliness may be gone."

In the episode, airing Tuesday at 8/7c on CBS, Tony is forced to face some tough truths about his father, played by guest star Robert Wagner. Weatherly and executive producer Shane Brennan say Tony's sense of humor comes from his father.

NCIS: Robert Wagner to play Tony's dad

"You can't help but smile when you see these two guys together, because Tony DiNozzo is such a funny guy," Brennan says. "When you meet his father, there's a twinkle in DiNozzo Sr.'s eye, and that's the same twinkle that Tony's got."

But Weatherly says humor might be the only thing Tony shares with his father. "Tony is pretty straightforward; he's not a liar," Weatherly says. "He tells the truth, and that is maybe where Tony and his father aren't the same. That is the big separation there. I think Tony is very similar to him, but in the same way he's a contrarian."

Landing Wagner for the role was a bit of a coup. Weatherly portrayed Wagner in a TV movie about Wagner's late wife, The Mystery of Natalie Wood. Brennan says there was never any other choice for DiNozzo Sr.

NCIS: Get ready for a family affair

"We were talking about doing an episode about Dinozzo's father, and it was instant: Robert Wagner has to play the dad," Brennan says. "He was an obvious choice physically. But there's something about Robert Wagner. The guy is such an icon, and [it was] a privilege to have him say yes — and he didn't just say yes, he's embraced the show and become an ambassador for the show."

Wagner says his favorite part of playing the recurring character was revealing new things about Tony. Viewers will learn that Tony's mother died when he was only 8 years old, and that the money Tony was raised on might not have been earned legitimately. Dinozzo Sr. might even be a bit of a con man.

"He's a very in-and—out kind of guy," Wagner says when describing his interactions with the other members of Tony's team. "DiNozzo Sr. is a very interesting man. He's an attractive guy and [the team] see aspects of him that interest them. He uses it all; he uses everything he can to make it work."

Rena Sofer keeps NCIS secrets by not knowing what they are

Wagner's character quickly gets involved in the case of the week. "He arrives unexpectedly, he becomes involved in the case, and he becomes an integral part of the case in a very unexpected way," Brennan says. "But the story goes much deeper than that. It really does play to the relationship that Tony has with his father that goes back to when he was just a little boy. And there's a bittersweet element to it.

"To me that's the most important thing," Brennan says. "The thing that makes NCIS different from all the other procedurals is the characters. When I took over the show, we said, 'Let's open that up. Let's shift the show a little and start revealing more about these characters. What you've now got is a show that started life as a procedural, but these characters have just exploded."