Sarah Michelle Gellar Guest Stars in 'Big Bang Theory' Series Finale: How This Surprise Happened! (Exclusive)

"Is that Buffy the vampire slayer?!"

Why yes, Sheldon Cooper. Yes, it is.

On Thursday night’s series finale of The Big Bang Theory, fans were gifted with a TV star treat when actress Sarah Michelle Gellar made a surprise appearance in the CBS comedy’s very last episode.

Gellar, who guest starred as herself, accompanied Raj Koothrappali to Sheldon and Amy’s Noble Prize acceptance ceremony in Stockholm, Sweden. Apparently, Sarah and Raj met while they were airplane seatmates on the long flight from Los Angeles and she graciously agreed to be his plus one to the big event.

"This is not a date," Sarah stressed to a gleeful Raj while the audience laughed.

The long-running series has previously referenced Gellar’s vampire-killing character on a number of episodes, but just how did The Big Bang Theory pull off this final guest star surprise?

Only ET has the exclusive behind-the-scenes secrets from executive producers Steve Molaro and Steve Holland. (Who also just so happen to be BIG Buffy fans!)

ET: It was so much fun to see Buffy the Vampire Slayer star Sarah Michelle Gellar appear in Big Bang’s series finale. How did you make this happen?

Steve Molaro: We knew that they were going to be on the plane and that there was no one sitting next to Koothrappali. Then [Big Bang creator] Chuck [Lorre] was like, "This seems like an opportunity for him to be sitting next to somebody fun. Maybe there's a celebrity that we can incorporate into this."

Steve Holland: I mean, Steve and I are both big Buffy fans  -- as are a lot of people in the [writers'] room. And rather than going for whoever is the hot “right now” celeb, it seemed like a really cool, nerd-pull for these characters who have been fans of Buffy forever too. And it was our way to get to meet her before the end. [Laughs]

How did you ask Sarah to guest star in the episode? Did she say yes right away?

SH: Well, Chuck spoke to her, but it’s always a good sign when the first thing people say is, "Our family are huge fans of this show!" That was how it went with Sean Astin too. We reached out and he’s like, "Our family are gigantic fans, we watch every week!" So then it’s like you’re pretty safe and then it was just a matter of working out scheduling [with Sarah.]

It was such a fun moment when Sheldon realized that actress Sarah Michelle Gellar was sitting in the audience while he was accepting a Nobel.

SM: That was a genuinely stunned and confused look on his face as he said, “Is that Buffy the vampire slayer?” in the middle of his Nobel speech. It was so great.

In the episode, Sarah made it a point to tell Raj that they were not on a date. Why include this line in the moment?

SM: It seemed funny. We know in real life she’s married to Freddie Prinze Jr. and, obviously, it’s a TV show, but it was just a funny way to cover our bases and get a joke in and give her a good laugh along the way.

How did you keep Sarah’s appearance a secret?

SH: Well, I mean, we filmed it the day before [audiences] saw it in the live show, so we didn’t have to keep it a secret for long. Actually, we shot it that morning.

SM: But we had to keep telling everybody on set and on the cast, "Please be careful on social media and please be careful not to post pictures."

For more behind-the-scenes secrets about The Big Bang Theory’s series finale, check out ET’s exclusive content below!

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