Jason Alexander reveals why Susan was killed off in 'Seinfeld'

They may have been a cute couple on screen, but Jason Alexander revealed why working with his Seinfeld co-star Heidi Swedberg wasn't always "serenity now."

Alexander appeared on The Howard Stern Show on Wednesday and discussed why Swedberg's character Susan Ross, the fiance to Alexander's character George Costanza, was killed off the show.

WARNING: explicit language

"I love [Swedberg]. She's a terrific girl. I love her ... I couldn't figure out how to play off of her," Alexander told Stern. "Her instincts for doing a scene, where the comedy was, and mine were always misfiring. And she would do something, and I would go, 'OK, I see what she's going to do, I'm going to adjust to her,' and then I'd adjust, and she'd change," Alexander said.

The seventh season of Seinfeld featured George and Susan becoming engaged, but with George trying to sabotage the relationship. Eventually, in the season finale, Susan died after licking toxic poisonous envelopes putting together the couple's wedding invitations.

"I had done three episodes with her, and Larry calls me up at the beginning of the season and goes, 'Good news: I've got a great arc for you this season - you're gonna get engaged.' 'Oh, that's great! Who do I get engaged to?' 'Susan.' And I went, 'Oh, great. Who's playing George?' Because it was such a disaster," Alexander said.

Alexander said he had previously expressed his frustrations to co-stars Jerry Seinfeld and Julia Louie-Dreyfus and series co-creator Larry David.

"Julia said, 'I know, don't you want to just kill her?' And Larry went, 'Kebang,'" Alexander said.

"Every time I tell this story, I cringe," Alexander admitted to Stern. "Heidi is the sweetest."

On Thursday, Alexander tweeted apologizing for the remarks he made about Swedberg. Alexander said in the apology that "is a kind, lovely person who undoubtedly worked really hard to create Susan" but that he was "always concerned that it wasn't working."

Following her run on Seinfeld, Swedberg appeared in television shows like ER, Gilmore Girls, and Bones, with her most recent role in 2010 on the cable series Hawthorne. Now Swedberg lives in Hawaii, where she plays and teaches ukulele music and is the lead performer in the Sukey Jump Band, according to ABC News.

Alexander has appeared in several films, television shows and theatrical productions after Seinfeld ended. The actor will taking over Larry David's role in the Broadway comedy Fish in the Dark on June 9.