Castle on a storm cloud? Following ABC's stunning announcement on Monday, April 18, that Stana Katic is exiting the popular procedural Castle, a source tells Us Weekly exclusively that the actress and longtime costar Nathan Fillion have been having behind-the-scenes friction.

Katic (NYPD Captain Kate Beckett) is leaving the show after its currently airing eighth season, and ABC released a statement praising her "talent and dedication to the series." She and Fillion (mystery novelist Richard Castle) both originated their roles in the March 2009 pilot of Castle, which has not yet been picked up for a ninth season.

ABC also announced that Tamala Jones (Lanie Parish) will exit after season 8.

But while the two lead characters — who tied the knot during a season 6 episode in 2014 — have a strong bond, the same sadly cannot be said for the stars themselves.

"Stana Katic and Nathan Fillion completely despise each other," the source tells Us. "They will not speak when they are off set, and this has been going on for seasons now."

"This season, it got so out of hand they made Stana and Nathan go to couples counseling together," adds the insider, who says that the actress has been in tears on the set because of the tension.

Katic's rep tells Us that the actress "had absolutely no issues" with her costar. Fillion's team did not immediately respond to a request for comment.

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