Sean Maher previously appeared as Simon Tam on Firefly (2002) and the movie Serenity (2005) with Nathan Fillion, who played Malcolm Reynolds.
This is the first appearance of the character of prison inmate Oscar Hutchinson (played by Matthew Glave), who is the only guest star to appear in all four seasons of the show.
In addition to appearing on Firefly (2002) together, Nathan Fillion and Sean Maher both appeared in Joss Whedon's Much Ado About Nothing (2012) as Dogberry and Don John respectively. Coincidentally, Fillion played a detective/police-like character and Maher played the antagonist in both this episode and Much Ado About Nothing.
Nathan Fillion's character asks an author to sign a copy of her book. In Castle (2009), Fillion played an author frequently being asked to sign his books. This may have been a deliberate nod.
Sean Maher charactor's name is Caleb, the lead Nathan Fillion played a character named Caleb in Buffy the Vampire Slayer. Additionally, Buffy was created by Joss Whedon, as was Firefly which starred both Nathan Fillion and Sean Maher.