Farewell to 'SPN' Day 14: James Marsters' 'SPN' Moment & Misjudging His 'Buffy' Co-Star

Jack Rowand/The CW

Hey there, SPN fans! Join us in our 15-day countdown to the series finale of Supernatural, featuring a look back with the cast (and guest stars!) at 15 seasons of demon-hunting and apocalypse-preventing, as well as exclusive content, sneak peeks, and more.

Supernatural's long list of guest stars could warrant enough stories for months, even years, but in our final SPN Moment spotlight before this week's grand finale, we're looking back to a 2011 episode, "Shut Up, Dr. Phil," which featured a Buffy the Vampire Slayer reunion between former costars James Marsters and Charisma Carpenter, who played Buffy's Spike and Cordelia, respectively.

These two were far from the first Buffyverse actors to appear on the series (Hello, Jeffrey Dean Morgan) and wouldn't be the last (Felicia Day, among others), but they did make an impact playing the bewitching (pun intended) couple, Don (Marsters) and Maggie (Carpenter), whose marital spat rose to magical proportions, leaving hunters Sam and Dean Winchester (Jared Padalecki and Jensen Ackles) to clean up their mess.

"The first thing that comes to mind when I think of my time on Supernatural is what a happy set it was," Marsters, who has also appeared on Angel, Smallville, Marvel's Runaways, and more genre-packed series, says of his brief time on Supernatural. "They had been making the show for a handful of years, and by that time cast and crew can grow weary. I asked the crew why this was [not the case] and was told that Jared and Jensen were both a joy to work with, and everyone felt lucky to be on the show. To me, it felt like working at a summer camp."

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Plus, take a look behind the scenes with Jared Padalecki and Jensen Ackles.

"I had been told that if Jared and Jensen liked you they would prank you, and on my final day I found out that I was OK in their eyes," he notes. While shooting a close-up, during which the director was in another room, watching just the monitor, and unable to see anything "off-camera," J2 made their move.

"During my close up I felt a light 'bobbling' up above my thighs, so to speak. As I was talking, I glanced down and saw that Jared was standing on one foot with his shoe off, and was the 'bobbler.'" Marsters says. "His expression seemed to say, 'Now what are you gonna do?'"

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Stars Jared Padalecki and Jensen Ackles also weigh in on the tearful conclusion.

Marsters didn't mind at all. "I was game," he says. "I finished the take, and the director called out, 'Cut, print, moving on,' and Jared and Jensen both cracked up, bowed their heads, and said, 'Dude! You’re unbreakable!' I was proud. 'Unbreakable' is a high compliment for an actor."

The warm and fuzzy feelings from the cast and crew weren't Marsters' only realization that week. "It was the first time I realized that Charisma Carpenter was a nice person," he exclaims. "I had worked with her on Buffy for years, and she had me entirely fooled. She’s such a good actor that I saw her as Cordelia! We rarely were in the same scene. When we were, Cordy was fleeing from Spike, so our chairs were usually on opposite sides of the set. I never discovered that she’s an absolute sweetheart. I remember talking with her on set between takes, and gasping saying, 'Oh my God, I’m so sorry. I’ve been judging you for years!'"

Stay tuned for more Supernatural fun as TV Insider counts down to the November 19 series finale.

Supernatural Series Finale, Thursday, November 19, 9/8c, The CW