The Chuck actor will appear in tonight's (April 16) episode of the ABC series, in which his former colleague stars as the eponymous character.
"To have that natural chemistry that we had from eight years ago, to look at Nathan's face and 'get' exactly what he's thinking, was great," Baldwin told TV Line.
"[Firefly] fans have been prodding us to share some scenes. Anytime I get to work with anyone from that universe is a 'resurrection', if you will."
Baldwin added that his Castle episode will include "subtle" nods to Firefly, "but nothing overt".
Teasing the cameo further, showrunner Andrew W Marlowe said Baldwin's character Detective Ethan Slaughter could become Castle's "new muse".
"He takes Castle on quite the ride," he continued. "Maybe it's the most exciting case of his life, or maybe it's 'The grass is always greener'. That's the journey he goes on."
Battlestar Galactica actor Tahmoh Penikett has also landed a role in Castle and will appear in the season finale on May 7.
The show airs Mondays at 10/9c on ABC.
> 'Castle' creator Andrew W Marlowe teases season finale