Behind the Scenes of the 'Frightening' 'NCIS' Season Finale

No one is safe on tonight's season finale of NCIS!
"We're going into a very disturbing territory on NCIS at the end," Pauley Perrette tells ET. "What's happening here is really going to stick with people. It's a terrifying thing."
In the finale, titled "Neverland," the NCIS crew ardently investigates a global terrorist group recruiting teens via the Internet after a deadly bombing overseas results in an emotional and surprising loss for the team.
While the tone of the show is serious, the mood on set stays lighthearted, which makes for some great behind-the-scenes footage.
"We always have an outstanding NCIS gag reel, which no one gets to see but us," Pauley said. "If there is a gag reel, 20 minutes of it is Michael Weatherly."
Watch the video to see Michael in rare form, goofing off on set.
NCIS airs Tuesdays at 8 p.m. ET/PT on CBS.