Sonja Flemming/CBS
Scott Huver
September 27, 2015 02:50 PM

After 15 seasons, 337 episodes, countless autopsies, several cast changes, three spin-offs and some of the most inventive murder investigations in TV history, CSI: Crime Scene Investigation is ready to close the morgue drawer for good.

As the now-iconic series airs its final two-hour installment on Sunday, several longtime cast members – including original star William Petersen, who returns as Gil Grissom for the finale – gathered recently at the Paley Center for Media in Beverly Hills to look back on the fond memories, famous fans turned guest stars (Justin Bieber and Taylor Swift among them) and the creepy forensic details they’ll never forget.

CSI stars Jorja Fox and William Petersen at the PaleyFest 2015 Fall TV Preview
Frederick M. Brown/Getty

Petersen, 62, remembers recognizing the remarkable spark happening behind the scenes with the cast and creators even as the show caught fire out of the gate with audiences, pushing boundaries of graphic content and reinventing the crime procedural as it become one of the fourth-longest-running and one-time highest rated series on television.

“The first year of the show was pretty remarkable in terms of the creative energy that was displayed on a daily basis from everybody that worked on it,” Petersen tells PEOPLE. “It was very group-created. And being a part of that first season and putting that together was the most creative time I’ve ever had in my life. It was exhausting, but it was special.”

Beyond being entranced by the newly sophisticated level of high-tech forensic investigation and the clever murder scenarios the show employed, fans also fell for the character’s personal dramas – most specifically Grissom’s slow-simmering romance with Jorja Fox‘s fellow investigator Sara Sidle.

“All we wanted to do was sort of have a real relationship that exists sometimes in the workplace where there’s a little thing that happens,” says Petersen. “And that becomes a little bit more of a thing. We never wanted to have a romance. We didn’t want to stop the show for that. So what was fun was finding the little ways we could fall for each other.”

“The fact that the audience who did love these stories and mysteries would actually care a little bit about the personal lives of these characters was an even more gigantic compliment,” says Fox, 47, admitting the series “probably changed my life in every way conceivable . The stuff I’ve gotten to do and the places I’ve gotten to go, and the dark places and the high places. I got to get married. Almost died, actually nine times. I was literally out of lives – like, ‘I’m a cat! I need to get off this show right now because I’m done!’ It’s been incredible.”

CSI‘s creator Anthony E. Zuiker remembers how, along with launching the early careers of guest actors like Dakota Fanning, Jeremy Renner, Rumer Willis and Kellan Lutz, several celebrity fans wrangled appearances on the show, including emerging pop star Bieber, who played a serial bomber in two 2011 episodes.

Justin Bieber (as Jason McCann) on CSI
Sonja Flemming/CBS

“He was huge back then,” recalls Zuiker. “We brought in a bunch of new viewers for us during that time. We don’t really get starstruck in the business, but it’s just really fun to kind of have somebody that huge on the globe to come do the show.”

The teen idol also managed to terrorize the CSI team with a non-stop series of behind-the-scenes pranks, says Zuiker. “He locked our line producer in the closet. He put his hands on a crew cake and [squished the frosting]. He was riding a skateboard while holding on to the tram at the studios. He came to sign his contact without his parents there with two friends from Canada. I know that he was a little lippy here and there. We definitely did not want him back. He had to apologize to the crew when coming back, which he did. He left in good graces, but boy, we didn’t like him for a bit.”

Swift also found her way into the lab, playing a murdered teenager. Jon Wellner, who started as a researcher for the series and later joined the cast for several seasons as toxicologist Henry Andrews, remembers Swift’s stint as far less chaotic than Bieber’s.

“I went into the makeup trailer, and she was there – and you know, you don’t want to be a fan,” recalls Wellner. “She’s getting her makeup done, and she jumps out of the chair and said, “Henry! That’s my character!” The fact that Taylor Swift knows who I am? I made it! She was with her mom, just so happy to be there, with the perfect attitude. I thought she did an amazing job. If you saw the episode, she really kills it. And that’s what our show is all about. We’re all working together to make a great show. We’re all very professional, but we can all have fun and be friends. We’re all friends outside of the show.”

Taylor Swift (as Haley Jones) on CSI
Monty Brinton/CBS

Cast members admit there are certain gruesome forensic details they learned on the show that will remain forever burned in their brains. “That maggots can help us identify a victim because they transfer some of the DNA from the bodies they’re chomping on,” says Eric Szmanda, who played DNA specialist Greg Sanders, with a shudder. “That’s definitely one that will stick with you.”

“If I had a dollar for every time I’ve said “blunt force trauma,” chuckled Robert David Hall, who played Chief Medical Examiner Al Robbins through the show’s entire run. Hall recalls spending time coroners in Los Angeles and Las Vegas to familiarize himself in autopsies because “I wanted my character to appear to have done this 10,000 times before. Believe me, I didn’t know anything about forensic medicine. So I’ve immersed myself in it as much as you can, and still not have a medical degree.”

Petersen says he’s pleased that the finale allowed him to reunite with former colleagues – including Marg Helgenberger, George Eads and Wallace Langham – as well as subsequent cast members like Ted Danson and Elisabeth Shue. “We thank CBS for giving us a chance to say goodbye to our fans. And hopefully, we’ve done a good enough job that they will feel that we have said thank you.”

“I would have done it anyway because I wouldn’t have missed it,’ says Fox. “[But] it really, truly, is, I think, a love letter to our fans. I really hope they enjoy it. It’s amazing.”

The finale promises “a lot of tears, a lot of warmth and satisfaction of our journey,” says Zuiker. “I’ve been afforded to do things and have experiences, work with great people and actors I would have never had the chance to without the success of CSI. We’ve traveled the world together, we’ve made great motion pictures together. I think when we’re all long gone, I believe CSI will still rerun and be honored and cherished like Twilight Zone and I Love Lucy. That’s great company.”

RELATED VIDEO: That Settles It: Marg Helgenberger Had the Worst Pre-Fame Job Ever

CSI ends its run with a two-hour special, Immortality, airing Sunday at 9 p.m. ET on CBS.

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