Who put a big, furry rat in Ziva's (Cote de Pablo) desk? ET knows!

The 'NCIS" set is a happy one. The series is up in the ratings, which is unusual for a show in its sixth season, and there is a whole lot of punking going on!

"Mark Harmon has decided there is a little stuffed rat that he is going to put into Cote's desk whenever he can and, to our merriment, she is constantly surprised. It is the same joke every time. Fresh response," Michael Weatherly tells ET's Thea Andrews, who is visiting the set of the hit military crime drama. "He is the biggest prankster, but when asked by him, I will swear that is not true."

Thea gets confirmation on the rat story from Cote, who says, "He enjoys me screaming."

Cote says she will get back at Harmon someday, but so far she hasn't come up with a good enough scheme.

On tonight's episode of "NCIS," a top secret war game leads to very real consequences for one of the team members.

"Michael and I run around and get caught by all these soldiers," Cote explains. "There was an incredibly big fight scene. I had to take out all these guys. It was the highlight of the day. I take out five guys and it takes a girl to take me out, which I kind of like. This girl comes in and is blonde, tall and huge and [she punches me] and I am out."

Then there is another moment in the episode where Ziva and DiNozzo (Weatherly) get intimate.

"I think the audience likes that storyline," Cote says of the sexual tension between her character and Michael's. "I think we will tease the [audience] more. We just keep sprinkling every scene with bits of flirtatiousness or anger or jealous. That is a fun dynamic to play with."

"NCIS" airs tonight at 8 p.m. on CBS.