Harry Shum Jr - who plays dancer Mike Chang in the Fox comedy - explained that the Star Wars fan favourite appears in a dream of Kevin McHale's character Artie Abrams.
"It was incredibly awesome," Shum Jr told AOL TV. "It was a huge surprise.
"He makes an appearance into Artie's dream in a way. I can't really delve into the details, but it's something to do with Artie."
The actor added that his Glee co-stars and crew made the most of having the Wookie on set. "Half of us came in right after he was done and it was like a mad house," he said.
"Everyone's face was so happy. They were showing off their pictures. They actually got him to come back to take pictures with us, too.
"It was so funny how many people actually love Chewbacca."
The Glee cast covered Mariah Carey's 'All I Want for Christmas is You' for the episode and subsequent second Christmas album.
The show airs on Tuesdays at 8/7c on Fox in the US and on Thursdays at 9pm on Sky1 in the UK. The Christmas special episode will air in the US on December 13.
> Exclusive: Glee streams Volume 7 soundtrack online - listen