150 episodes. It's a milestone for any series and any series star.

But the Criminal Minds installment airing tomorrow night is extra special for A.J. Cook, considering she didn't think she'd be a part of it following a cast upheaval in 2010.

"For me, this one means a little bit more," the actress told me over the phone. "I'm so overwhelmed even to this day by all the fan support. Not a day goes by when I'm not grateful for it and grateful to be working with this group of actors. And here we are..."

Here they are, indeed, set to unleash "Unknown Suspect" on the world.

As previewed above, the episode will focus on a serial rapist (known as "The Piano Man" and played by Jay Karnes) who resurfaces in order to re-assault previous victims. But Cook makes it sound like the case, which takes the BAU to Houston, will be anything but typical.

"The best way to say it is: expect the unexpected," the actress teased. "It will be a roller coaster of a ride for the audience to go on, with an interesting twist that [Karnes] handles so well."

Cook points to the show's 100th episode and to her character's "swan song" episode as two that really stand out over the years, but it's clear that any time spent with her castmates is welcome, no matter what they're shooting or doing.

"I've been texting with the girls all night and morning," she said, joking: "I'm almost afraid of my next job now! What if I end up with all these crazy people? It really can't get any better than it is now."

The same might not be true for JJ, however. Cook is really excited about an upcoming episode in which we see JJ "in a completely different light."

"There's a pretty ugly fight," the actress previews. "You see a tougher, scrapper, bloodier JJ. It was so much fun to film and I can't wait for everyone to see it."

Criminal Minds airs the big 1-5-0 tomorrow night. Visit TV Fanatic soon after it concludes for a detailed review of events.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.