"NCIS: Los Angeles," starring Chris O'Donnell and LL Cool J, is one of the most highly anticipated series of the new fall season. Now ET has the inside 411 from the set of the CBS spinoff.

"NCIS" star Pauley Perrette, who plays quirky but extremely talented forensic scientist Abby Sciuto, will cross over and appear in the second episode of the new season. Exec producer Shane Brennan says, "She does a crossover and she's directly involved with helping us solve the case."

Rocky Carroll, who plays NCIS boss Leon Vance, will be doing double duty on both series. Of course his character is in charge of the entire Naval Criminal Investigative Services, not just the Washington D.C. branch, so it makes sense.

It also makes sense because Leon has a past relationship with the new character that Oscar-winning actress Linda Hunt will play. Hunt will head the L.A. operation and ops center, which has had to move into new facilities -- a Spanish mission -- because their location was compromised in the episodes that aired as part of "NCIS" earlier this year, when Callen (O'Donnell) was shot and lay dying at the end.

Brennan says the new series will be familiar to "NCIS" rather than similar in that it will have the same mix of drama, emotion and humor, but from that point on, everything else is different. It is on the West Coast and there's a whole different vibe here. So he's playing those differences.

When the series opens, it is four months after the shooting and it is Callen's first day back on the job. "We see his scars. We literally see his scars," Brennan says. "Will we answer what happened to him and how it happened and why? In the very best tradition of 'NCIS,' yes. And in the very best tradition of 'NCIS,' you'll just have to wait. It will be a sweeps episode. I'll give you that."

Also, Brennan reveals that there's a deep backstory between Callen and Sam (LL Cool J) that will slowly reveal itself.

On the lighter side, LL reveals, "For me, the biggest challenge in this whole situation is just staying away from the craft service table. If I could just do that. The candy bars, the doodles, the snicker doodles, and sugar cookies -- it just drives me insane."

And new cast member Adam Jamal Craig is a techie in real life, so he will be the one person in the cast who actually understands the high tech equipment in the ops center.

"The ops center is really cool because, when you walk in there, like everything changes," LL adds. "The shutters close. There's all this Lockheed Martin technology in there. There's a lot of perceptive pixel, you know, screens and huge monitors. It's like a really cool place."

"NCIS: Los Angeles" premieres Tuesday, Sept. 22 at 9 p.m. on CBS, following "NCIS."