George (Jason Alexander) and Susan (Heidi Swedberg) were an item for most of two seasons before Susan was suddenly killed off. Alexander revealed the reason why in a new interview. (Photo: ScreenCap)

Jason Alexander, who played the hapless George Costanza on the hit ’90s sitcom “Seinfeld,” has finally answered one of the long-running show’s enduring mysteries. Why did George’s fiancee Susan die suddenly on the eve of their marriage? You’d be surprised to hear why.

Alexander appeared on the Howard Stern show on Sirius Radio yesterday (June 3) and finally revealed the secret behind the character’s death, nearly 20 years after the show ended.

Turns out, the entire cast, Jerry Seinfeld, Julia Louis Dreyfus and Michael Richards were in on the move.

You may recall, Susan Biddle Ross, played by Heidi Swedberg, was an NBC executive. She first appeared in the season 4 episode “The Pitch” when Jerry and George tried to pitch a sit-com pilot about their lives. She had a recurring role throughout the season.

When the season ended, she and George had broken up and she had taken a lesbian lover. She returned in season 7 and George proposed to her thinking that her lesbianism would lead her to say no. But she agreed, leading George to utter the famous line that her lesbianism “didn’t take.”

Most of the seventh season included a Susan and George plot line as he tried desperately to get out of the marriage to no avail. He was tasked with ordering the wedding invitations and picked out the cheapest ones available. As it turns out, the glue was toxic.

Susan died and was written out of the show, although George had recurring scenes with her parents and was put in charge of Susan’s charitable trust.

“The story goes that she was killed off because you couldn’t stand working with her,” Stern challenged.

“This poor girl,” he said, “I love her. I couldn’t figure out how to play off of her,” he revealed. “Her instincts for doing a scene — where the comedy was — and mine were always misfiring.”

Jerry and Julia thought he was crazy, but after working with her Jerry said it was fu*King impossible!”

“Julia actually said, ‘I know, don’t you just wanna kill her?'”

Show co-creator and head writer Larry David picked up on Julia’s remark and brought her back for the seventh season to do just that. “Every time I tell this story I cringe because Heidi is the sweetest,” Alexander said.

Check out the audio from Stern’s show, let us know your thoughts and be sure to follow IM on Twitter for the latest television news.