It's no secret that Castle co-stars Nathan Fillion and Stana Katic don't get along. Though they play the two lead characters on ABC's hit show, Katic and Fillion have had fewer scenes working together in recent seasons at their own request, and Katic isn't invited back for a possible ninth and final season. This is quite the departure from earlier seasons, when the two actors got along and were even rumored to be dating.
According to Dish Nation, the reason Stana Katic and Nathan Fillion grew apart was due to Stana's rising star, which caused Nathan to feel upstaged as he considered himself the star of the show. One insider said "As her popularity increased, she was given pay raises, and she even got a new dressing room equal to his. As time went on, Nathan became more and more distant, insisting that they only set foot on set together when it was absolutely necessary."
In one incident, Stana went on the set to "shadow" the director on her day off because she aspired to become a director and wanted to learn the ropes. Nathan was livid and demanded she leave. “It was Stana’s day off and she opted to come in just to follow the director around, but when Nathan found out her intentions, he put his foot down and insisted she go home.”