Perrette's character, Abby Sciuto, will appear in a "fun little cameo" and help solve the case, Brennan said at the show's Television Critics Association fall press tour panel.
A tour of the show's set revealed another previously unannounced guest: Mathew St. Patrick, who played Six Feet Under's Keith. His face appeared on a video screen in the show's operations center, and Brennan told TVGuide.com he'll play "a former special ops guy who actually gets deeply involved in the case and may or may not be a bad guy."
St. Patrick's character may return, Brennan says: "Let me put it this way: He lives."
He also hinted about how the NCIS storyline that ended with Chris O'Donnell's character, Special Agent G. Callen, taking a bullet will continue on the spin-off. "It's four months later and it's Callen's first day back on the job and we see his scars," Brennan said. "We literally see his scars."
He said what happened on NCIS will also be explained — eventually.
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The panel's other highlights included the sight of the very large LL Cool J sitting next to the very small Linda Hunt — and the rapper/actor gamely playing along when a reporter asked if his role was a comeback. "Don't call it a comeback," he joked.