At first blush, "Castle" and "The Mentalist" seem like the same TV show. They both feature strong-willed, brunette women who have to solve crimes as an annoying civilian preoccupied with his own agenda tags along. Despite the similar premises, each show has a distinct tone and style.
"Castle's" wealthy, popular playboy Rick Castle is a divorced father and successful mystery novelist who became involved with the New York Police Department after a copycat murder mimicked one of his books. Castle's penchant for developing fictional criminals makes him a natural at solving crimes. Working alongside the NYPD ended his writer's block and inspired a new book series.
On "The Mentalist," Patrick Jane developed his powers of observation and deductive reasoning while reluctantly working cons alongside his father. As an adult, he feigned psychic abilities and personally offended a notorious serial killer, Red John, during a TV appearance. He found his wife and daughter murdered as a result. Jane dedicated his life to catching Red John and worked his way into the California Bureau of Investigation (CBI) as a consultant. Although he helps the CBI with various cases, he is very clear that his ultimate goal is to stop Red John at all costs, even his own life.
Castle has a unique talent for connecting the dots and discovering the criminal's motives. He shares his thoughts with the team, sometimes talking about book ideas in the process. He and Detective Kate Beckett work closely and often find themselves alone in dangerous situations.
Jane is cocky, confrontational and enjoys engaging criminals on a personal level. Although he is an expert at reading body language, he lacks people skills--perhaps because little else matters after he caused the death of his family. His powers of observation allow him to notice seemingly insignificant details that ultimately prove vital to solving the case. Jane smugly works above his team, often breaking the rules to conduct an independent investigation, but his charm and skills keep him out of trouble.
Castle's Love Connection
Detective Beckett and Castle have a complicated relationship reminiscent of Maddie and David on "Moonlighting." Their playful, flirtatious interactions took a definitive turn in Season 4 after Castle proclaimed his love to a dying Beckett. She recovered and does not seem to remember the event, but their relationship subtly changed as she now reciprocates Castle's little comments and double entendres.
Jane's Love Connection
Although Jane antagonizes Agent Teresa Lisbon, the two genuinely care about each other. Jane is somewhat flirtatious, but their relationship seems more like that of a sister and her bratty little brother.
Overall, "Castle" has a lighter, sexier style with hints of film noir. Castle's happy home life and success offer a warm respite from the gory crimes. "The Mentalist" has a smug sense of humor and focuses on Jane's witty ability to outsmart criminals. His coworkers provide a loving surrogate family, but the darkness from his loss is always near and it feels as though he could go over the edge at any moment.