It’s one of our favorite “Seinfeld” episodes. You know, the one where Susan dies.

Well, it turns out the real reason co-creator Larry David killed her off wasn’t because the storyline had played out -- but because the other stars found it impossible to click with the actress who portrayed her, Heidi Swedberg.

And the idea to off her came from none other than Julia Louis-Dreyfus (Elaine).

Alexander (George) revealed all on the “The Howard Stern Show,” where he appeared Wednesday to promote his taking over for David on Broadway in David’s “Fish in the Dark.”

“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.

“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.”

At first, Jerry Seinfeld and Louis-Dreyfus had no problem with her, he said.

But soon, when they stumbled in their scenes with Swedberg, the group grousing started.

One night, at dinner (sans Swedberg), “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 said. “Heidi is the sweetest.”