Castle follows Nathan Fillion as Richard Castle, a famous mystery novelist who is initially called in to help the NYPD solve a copy-cat murder based on his novels. Stana Katic stars opposite as the young detective Kate Beckett. Following his encounter with Beckett, Castle decides to use her as the model for the main character of his next book series. He uses his contacts and receives permission to accompany Beckett while investigating cases. While technically a drama series, Castle also features comedy and romantic tension.
- Nathan Fillion as Richard Castle – a well-known author, who, after having just killed off his most famous character and being nagged by his publisher ex-wife, is pulled in by NYPD Detective Kate Beckett to help with a copy-cat case and discovers a new source of inspiration. Castle now has the opportunity to use his knowledge and skills to help solve murders.
- Stana Katic as Det. Kate Beckett – a homicide detective with the NYPD. She is a first-class investigator who has gained a reputation as a detective who’s intrigued by unusual cases. She joined the force after her mother was murdered and the case was never solved.
- Jon Huertas as Det. Javier Esposito – works in the homicide division with Kevin Ryan as part of Beckett’s team. He is always ready with a sardonic comment to keep Beckett on her toes and enjoys the way Castle riles her up. He and Ryan are friends, even though they constantly argue about obscure facts.
- Seamus Dever as Det. Kevin Ryan – also works alongside Esposito as part of Beckett’s team. He and Esposito regularly tease Beckett, but they are incredibly protective of her and ultimately treat her like one of the guys. The character was a late addition to the show, as the original presentation pilot was only 37 minutes long.
- Tamala Jones as Dr. Lanie Parish – a medical examiner who has a friendly outlook. As a friend of Beckett’s, she is one of the few people Beckett can talk to easily and without reservations. She is also one of the few people who notices the personal connection between Beckett and Castle, and urges her to act on it.
- Ruben Santiago-Hudson as Cpt. Roy Montgomery – Beckett’s boss who appreciates Beckett’s determination and diligence but maintains a close watch to make sure the investigation goes smoothly. He is amused by how much Castle annoys Beckett, but also appreciates how effective the two are when they work together.
- Molly Quinn as Alexis Castle – Castle’s fifteen-year-old daughter, who is often noted to be more mature and responsible than her father. She is very bright and enjoys school. Her social and interpersonal issues often parallel some aspect of the case Castle and Beckett are investigating.
- Susan Sullivan as Martha Rodgers – an actress on and off Broadway, is Castle’s mother and lives with him and her granddaughter Alexis. She helps her son through the difficulties of raising a teenage girl (even one as mature as Alexis), but is also shown to have an active social life. She lives with Castle partly because a previous husband absconded with all her savings.