Warning: The following contains spoilers for Season 4 of The Rookie, which is currently airing Sunday nights on ABC.

The Rookie is a singular title, making it seem like the series is overtly focused on just one character. Looking at the show today, however, that isn’t the current trajectory on ABC’s leading police procedural. Right now, The Rookie is driven by more of an ensemble cast. In all actuality, though, the contrast between the series’ title and its current look is a testament to how the show has grown over its first four seasons.

When The Rookie was introduced, it was tabbed as a show starring Nathan Fillion – the former leading man on Castle – as John Nolan. Based on the true story of Bill Norcross, Nolan's character was a forty-five-year-old man who decided that he wanted to be a police officer after inadvertently helping officers during a bank robbery. So, he moved cross-country from Pennsylvania to Los Angeles so that he could join the LAPD – one of only two agencies that accepted new officers over the age of thirty-seven.

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While Nolan was “the rookie,” the series introduced a few other rookies that went through training with him over the first three seasons. On top of that, various training officers were introduced over the first few seasons, along with a police captain and sergeant. While Nolan was clearly meant to be the main character at first, the series has naturally developed its ensemble characters to become a show that is truly driven by the people around Nolan just as much as the rookie himself.

The Rookie main characters Nolan, Chen and West sitting at a desk together

From a general perspective, it benefits almost any television show to have a well-developed cast. Stock characters around a lead make for a boring and flat viewing experience. As part of its progression into an ensemble cast, The Rookie features two or three different plotlines in each episode that often overlap and interweave. That makes things more realistic, and it allows characters to interact in natural ways.

RELATED: The Rookie Introduces a New Officer – and He’s … a Murderer?

Another benefit to having a well-developed cast is that it makes the show more relatable for viewers, and that’s something that The Rookie has improved upon since its first season. This is especially true of such a diverse cast. In its tenure, The Rookie has featured a woman in a leadership position, a woman as a detective, multiple Black men in leadership positions, along with a gay, Black man as one of the main characters. Diversity doesn't work well if they are only token stock characters, but since all of them have had a chance to develop as the show has gone on, viewers can more readily see themselves in various characters.

Currently, The Rookie is just beginning its fourth season, and it is still focusing on an ensemble cast. The series just finished sending the LAPD to rescue Angela Lopez from a drug queenpin in Guatemala. However, finding out where she was taken was problematic for Lopez’s fiancé. He had to promise to be a dirty lawyer for a career criminal in LA, and this promises to be a major storyline going forward.

To see more of Nathan Fillion and the brilliant ensemble cast around him, tune into The Rookie, with new episodes airing Sundays at 10 pm ET on ABC.

KEEP READING: The Rookie Season 4 Premiere Kills Off Two Major Characters