ET was on the set of "Castle" where Nathan Fillion and former "Dancing with the Stars" contestant Debi Mazar dish about what's to come on Monday night's episode of the hit ABC show!

Mazar was honored that a head of ABC thought of her for the part. She hopes this guest stint could turn into a recurring role. She told ET, "If they want to have me back and if Steve McPherson thinks I'm fabulous, then he'll work out the dates."

In the episode entitled "When the Bough Breaks," book agent Paula Haas (Mazar) approaches Castle (Fillion) with an extraordinary career opportunity that would mean an end to his relationship with Beckett (Stana Katic). Will this be their last case together? Find out when "Castle" airs Monday night at 10 p.m. on ABC.

Plus, watch the video for more on tonight's episode.