The actress who plays Ryan's wife in the show, Juliana Dever, is Seamus Dever's (Ryan's) real life wife.
When Stana Katic first met Nathan Fillion, she had just gotten out of hair and makeup for the final callback between her and one other actress, and realized she was wearing a blouse that was too long. She asked for a pair of scissors, saw Nathan Fillion getting coffee, and greeted him. He asked if she wanted to run lines, and she replied, "No, but speaking of lines, can you cut a straight line?" To which he replied "I can try." And so he cut her blouse shorter for her. The producers saw this and she got the part.
Detective Kevin Ryan (Seamus Dever) could be seen carrying a copy of one of Castle's books at least once in every episode.
Though they shared an engaging chemistry on screen, off-camera issues between Nathan Fillion and Stana Katic allegedly got so bad in the later seasons that they were required to attend couples counseling together. Their professional relationship had reportedly deteriorated to the point that by the final season, each refused to speak to the other except in character while cameras were rolling, staying in separate rooms between takes.