Katy Perry and the 9 best guest stars on The Rookie

The Rookie has solved some brutal crimes during its six seasons and will solve many more in the upcoming season 7. Whether the guest stars were at the center of these crimes or propelling the stories forward, they made the cases and plots absorbing and fascinating to unfold. Katy Perry was among the stars, as well as a Hollywood film veteran and a guest star whose previous role was a rookie but turned "gangster." Yes, that actually happened, but not in the way you're thinking.

Showrunner Alexi Hawley recently teased that more guest stars are possible for next season, which will be back to its regular, pre-strike rhythm with a full season order. All I can say is that I'm looking forward to the new storylines and seeing how the guest stars will be written into them. Who knows, maybe a few could return to reprise their characters, which will loop back to the previous seasons.

While it's still early to know who'll appear on The Rookie next season, why not revisit some past guest stars who made a prominent impression in their respective episodes? Here's our list of the nine best!

The best guest stars on The Rookie

Katy Perry

Character: Herself
Episode: 2x16 "The Overnight"

In "The Overnight," Katy Perry stars as herself and is on the set of American Idol. Not surprisingly, her fellow judges Lionel Richie, and Luke Bryan are also in the scene. Host Ryan Seacrest, naturally has a cameo as well. The episode is considered a crossover of the two shows, and the formula works, as it brings out the light in life as well as its darkness.

In Perry's scene, Lucy Chen (Melissa O'Neil) accidently auditions for a coveted spot to be on Idol. What most fans don't know, and I certainly didn't, was that O'Neill won Canadian Idol in 2005 when she was only 17 years old.

Kelly Clarkson

Character: Herself
Episode: 5x12 "Death Notice"

Another Idol winner, Kelly Clarkson, made a brief but hilarious appearance in season 5. Clarkson got stuck in a hospital elevator with the LAPD team in her scene, and as innocent as it sounds, the cops were there on a guarding mission. It was definitely a treat for Clarkson since she's a big fan of The Rookie and its cast. She got the role when the cast appeared on her talk show on her birthday, and Nathan Fillion, who stars as Officer John Nolan, asked her to be a part of it. It was a birthday present to remember.

Following her episode, Clarkson went behind the scenes, detailing the experience and its production. During filming, she revealed that she was nervous and feared she would fail. That wasn't the case, as Fillion thought Clarkson did "fantastic."

Frances Fisher

Character: Evelyn Nolan
Episode(s): 3x03 "La Fiera" and 3x04 "Sabotage"

Frances Fisher is a Hollywood veteran and is most likely best known for her role as Ruth Dewitt Bukater, the mother of Kate Winslet's character Rose, in the 1997 blockbuster Titanic. However, for two episodes in season three, Fisher portrayed Evelyn Nolan, the mother of Officer John Nolan. But don't expect Evelyn to be an honorable citizen and a protector like her son. She has a crime record and is quite famous as a drug trafficker and scam artist in Pennsylvania.

Evelyn and John's relationship is considered strained, and the latter cut all ties with her when she brought her crimes to Los Angeles. In season 5, Evelyn passes away, but instead of hosting a funeral, Nolan takes the easy route and has his mother cremated.

It's a striking polarization of these two characters, as John's a good guy while his mother harms people for her own gains. Yet, the arc between them truly highlighted their characteristics.

Emily Deschanel

Character: Sarah Nolan
Episode: 3x12 "Brave Heart"

Emily Deschanel played John's ex-wife Sarah, and the two (platonically) reunited in the third season to support their son Henry, who must undergo major heart surgery, albeit that the procedure has risks, including cardiac arrest. Despite their intense angst and moving on to different partners, the former couple still have a spark between them, just in sheer observation.

Sarah also gave John a layer of personal depth as a parent, and witnessing his character shift from policeman to saving lives to being concerned regarding his son's life was important.

What fans might not know is that both Deschanel and Fillion played crime authors in two other successful procedural series, Bones on the FOX and Castle on ABC.

Pete Davidson

Character: Peter "Pete" Nolan
Episode(s): 2x13 "Follow-up Day", 4x05 "A.C.H." and 4x22 "Day in the Hole"

You might have caught Pete Davidson in NBC's Saturday Night Live! or know him for his past relationships with famous women like Ariana Grande and Kim Kardashian, but for three episodes of The Rookie, Davidson portrayed John's younger half-brother, Pete.

Unlike John, Pete had brushes with the law, which makes the character polarization between the two challenging but unique. The brothers met for the first time at the police station, following the death of their father. Finding out he has a sibling was a lot for John. He and his father had been estranged so he never knew he had a brother. Despite their rocky introduction, John agreed to help Pete with his later troubles and the two establish a budding sibling relationship.

Davidson's portrayal of Pete was humorous with a mix of seriousness balanced by a little bit of sarcasm.

Frankie Muniz

Character: Corey Harris
Episode: 3x07 "True Crime"

Frankie Muniz was a child actor and starred in the popular FOX comedy, Malcolm in the Middle, where he played a boy genius in a family of average intelligence. On The Rookie, Muniz's character Corey, was a former child actor, but traded the fame for faith by becoming a cult leader. While I'd much rather prefer the "innocuous" occupation of actor, I love the way the writers connected the two series, and Muniz is given a darker shadow to play with after the light-hearted role that propelled him into the spotlight.

Corey is described as a "putz" by the LAPD, and he describes himself as a "prophet." He's accused of murdering one of his devout followers as they knew "too much" about him and his real intentions. Yet, the real murderer came forward, admitting their crimes.

If you know anything about cult leaders, then you know that they often have a doomed ending. We won't divulge significant spoilers, but let's say that Corey's ending wasn't ceremonial.

Alan Tudyk

Character: Ellroy Basso
Episode(s): 2x08 "Clean Cut" and 4x22 "Day in the Hole"

Alan Tudyk's character, Ellroy, had the dirty occupation of cleaning up after crimes were committed, but on the plus side, he became a good friend of John's. In the season 4 finale, "Day in the Hole," Ellroy proved that he was much more than a forensic cleaner when he took on a suspect with greater physical strength and build while on the scene with John and Bailey Nune.

Tudyk's appearances mark the reunion between him and Fillion, as both actors were costars on the one season, cult classic FOX sci-fi drama Firefly in 2002.

Molly Quinn

Character: Ashley
Episode(s): 3x08 "Bad Blood" and 5x12 "Death Notice"

Another reunion with Fillion took place when Molly Quinn guest-starred as the troubled woman, Ashley, who left a DNA trail during a botched kidnapping scheme, linking her to the notorious killer, Oscar Hutcherson (Matthew Glave), who's revealed to be her biological father.

Ashley has no unified nor close relationship with her father, as he financially takes advantage of her, in addition to having abandoned her mother before she was born. Still, Hutcherson didn't know that Ashley was his.

In "Death Notice," Ashley is in need of a kidney transplant and her father is her perfect reciprocant. The transplant is successful, but Hutcherson's ordered not to leech any more money from his daughter.

Molly Quinn played Alexis Castle, the daughter of Fillion's Castle character, Richard Castle, for all eight seasons. Alexis was portrayed as the "sweet, down-to-earth" daughter, while Ashley was a much darker opposite.

Will.i.am

Character: Himself
Episode: 1x19 "The Checklist"

Will.i.am, of the multigenre music group Black Eyed Peas, was just an innocent bystander in his The Rookie cameo. When John responds to a silent alarm in Will.i.am's yard, he is naturally curious since he is home at the time. As it turns out, an uninvited guest drops in, and Will.i.am delivers a hilarious but pressing response to the guest regarding why it's not cool to "stalk." The cameo added lightness and fun to The Rookie's otherwise disturbing and emotionally complex narratives.

Bob Morley

Character: Half-Life
Episode: 3x14 "Threshold"

After departing The 100 as Bellamy Blake, Morley played a drug-ring gangster called Half-Life in The Rookie's season 3 finale. His performance was bone-chilling, sinister, and downright evil, and not the good-hearted Bellamy that Morley played. Yet, it takes talent to deliver a raw audience reaction, and he excelled in that department. I was creeped out by Half-Life but also focused on his storyline.

For a brief recap, "Threshold" centered on Lucy Chen going undercover to capture the infamous drug trafficker, Sandra "La Fiera" de La Cruz." Chen's plan worked flawlessly and La Fiera is arrested, but later escapes.

Morley played a rookie cop named Tony Moretti on the Australian police drama The Strip in 2008 which made the characterization of Half-Life that much more antagonistic and scathing.

The Rookie seasons 1-6 are available to stream on Hulu.