But for fans already mourning the loss, PEOPLE has an exclusive sneak peek at Monday's episode, where Katic, 37, is seen in her element. In the clip, Richard Castle (Nathan Fillion) and Kate Beckett (Katic) interview the victim of a crime in the hospital – and though it's pretty obvious to the show's protagonists who may have done it, the man appears so good-natured that he doesn't want to believe it could have been a former employee of his.
The episode comes as fans continue to be stunned by the news. On Tuesday, Fillion, 45, broke his silence to wish Katic his best.
"Castle has been one of the greatest joys of my creative life, and I hope the show continues on for years to come," he wrote. "Stana has been my partner all this time, and I thank her for creating the character of Beckett who will live on for all of us as one of the greatest police officers on television. I wish her well, and have no doubt she will succeed in everything she pursues. She will be missed."
Katic also shared her thanks: "To the fans: thank you for your support. I love you," she tweeted Monday.
To the fans: thank you for your support. I love you.— Stana Katic (@Stana_Katic) April 19, 2016
Castle airs Mondays (10 p.m. ET) on ABC.