Development
Ruby was described as a "demon hunter" in press releases prior to her debut so that her true demonic nature would surprise the audience. The writers created Ruby to change the perception of demons into more of a grey area, rather than the prevalent belief of "black and white", "They're evil, we're good" approach previously used in the series. However, the writers also planned for Ruby to impact the brothers negatively by facilitating the story arc of Sam falling into evil—which had been set up in the second season, but without follow-through—and causing a fracture in their relationship. Knowing this, the writers were amused by fans questioning why they were "trying to make [Ruby] likable". Despite Ruby's overall betrayal of the brothers, writer Sera Gamble commented, "[Ruby] brought the idea that you can't just dismiss demons as things that need to be killed right away. They could be useful, and while fundamentally untrustworthy, there might be cause to trust them in a given situation."
Fearing that introducing the character as an "[accessory] to the boys" would hinder their chances of successfully integrating her into the series, the writers intended that Ruby should be "a character in [her] own right" and deemed her an antagonist "with [her] own interests and [her] own motives" rather than a love interest to Sam or Dean, which they felt had been their mistake in their introduction of the widely disliked Jo Harvelle in the second season. While they were not planning on a romance between Ruby and either of the Winchesters in the third season, however, they were open to the possibility in the future, with Kripke saying, "If the chemistry is there, and we see the sparks, and we want it to happen, and the fans want it to happen, it'll happen." Due to "protective and occasionally nervous" fans, Kripke meant to introduce Ruby in "small doses". Wanting fans to know the show would always be about Sam and Dean, and nothing else, he stated, "[Ruby and Bela are] there for important plot elements, but it's not the Ruby and Bela show, nor is it about the four of them cruising around in the Impala together. It's about the guys."
Cassidy originally auditioned for the role of Bela Talbot, but ultimately received the part of Ruby. As opposed to using traditional demonic abilities such as telekinesis, Ruby instead relies on conventional martial arts and her demon-killing knife. Cassidy trained in kickboxing alongside Bela's actress Lauren Cohan to be able to perform Ruby's martial arts skills, prompting her to attempt as many of the fight scenes as she could rather than rely on her stunt double. Before filming for the third season began, she and Cohan decided to watch previous seasons together to catch up on the show. Cassidy also prepared by looking to Sharon Stone's performance in the film Basic Instinct for inspiration due to Ruby's manipulative ways. As Cassidy explained, "[Stone's character] always has the power, and there's this mystery about her." Costume designer Diane Widas had Ruby dressed in dark colors so that she would blend into shadows, also giving her pleather jackets and narrow jeans to allow Cassidy to be more active. Because of Cassidy's height difference with the lead actors—she is 5'7" while Sam's actor, Jared Padalecki, is 6'4"—she had to wear tall, spiky high heels that at times made her lose balance.
Kripke cited budgetary reasons for Cassidy's departure after the third season. According to Cassidy, however, Warner Bros.'s uncertainty about what direction to take Ruby in prompted her to leave when the opportunity to star in the series Harper's Island arose. To "make the best out of a bad situation", Kripke and the writers planned for Ruby to take on a new host every few episodes for the fourth season. They believed this would "keep [viewers] guessing", and provide a "cool character that most shows don't have the ability to do". Auditions for an unnamed "love interest" were held to recast Ruby with a new actress, and Cortese was hired for the part. She was then informed that she would actually be playing Ruby. Before the first episode she was in aired, however, she was said to be playing "a small-town waitress" named Kristy who had become "romantically involved" with Sam after Dean's death. Cortese played the first of what was expected to be many incarnations of Ruby, but an impressed Kripke ultimately chose to keep her in the role because she "brought a lot of the different colors and vulnerabilities to Ruby that [he] was really looking for". Although Cortese viewed DVDs of Cassidy's portrayal, she tried to make the character her own at the producers' request rather than emulate Cassidy. She was not as concerned with how Ruby was received by the audience—stating "if people don't like her, they don't like her"—as much as she was with using her performance to "answer questions" about Ruby and her relationship with the Winchester brothers, such as why Sam and Dean were continuing to work with Ruby in the fourth season.
