Biggest Castle Actors Guest Starred in The Rookie, but Don't Expect Stana Katic There

But fans shouldn't expect to see Castle co-star Stana Katic appear on The Rookie any time soon - at least, not as long as Nathan Fillion's there.

Castle ended after eight seasons in 2016, two years before The Rookie premiered. It starred Nathan Fillion, formerly of Firefly, as novelist Rick Castle who tags along with the NYPD to research homicides.

Fillion produced the final season of Castle and has since been an executive producer on The Rookie, a series in which he stars as the LAPD's oldest rookie cop.

Both he and Alexi Hawley have seemed determined to reminisce on their first hit show. Hawley was a writer and producer on Castle and is the creative mind behind The Rookie.

Several supporting Castle actors have appeared in The Rookie. Molly Quinn appeared as Castle's daughter Alexis, then guest-starred in Seasons 3 and 5 of The Rookie. Jon Huertas and Seamus Dever (Castle's Javier Esposito and Kevin Ryan) have guest appearances in The Rookie; Huertas even directed a couple of episodes.

But Stana Katic, one of Castle's most important and beloved actors, has yet to make an appearance.

Katic played Kate Beckett, Captain of NYPD's 12th Precinct. Over the first four seasons, fans came to watch the relationship unfold between Castle and Beckett - a classic 'will they, won't they' thing that finally landed when the two shared a kiss in Season 4. They got together the next season, and by Season 7, they were married. The season finale showed their future with three kids.

But in reality, the two couldn't stand each other. Fillion and Katic apparently were not on speaking terms later in the show, despite playing on-screen lovers. They were even sent to co-therapy to improve their working relationship.

Near the end of Season 8, the studio told Katic that she wouldn't appear in Season 9 if they chose to renew the series. The excuse was budgetary reasons, but rumors emerged that it was because of her relationship with Fillion.

Weeks later, ABC canceled the series, ending the drama there. Neither Fillion nor Katic has publicly talked about their feud. Of her potential firing, Katic was understandably hurt but looks back on her time on Castle positively after all these years.

Though it's been nearly 7 years since Castle's finale, there's no reason to believe the relationship between Katic and Fillion has healed. Despite all of the connections between the two shows, it's unlikely we'll ever see her on-screen with Fillion during The Rookie's run.

Season 5 returned on ABC April 18 after a multi-week break, set for a May 2 season finale. It has already been renewed for Season 6.