The following contains spoilers for Big Mouth Season 6, Episode 10, "F**ked Up Friday," now streaming on Netflix
Big Mouth's latest season was focused on the growth of the show's central cast. In one case, Big Mouth even found a way to build on a largely comical thread from previous seasons of the show. Missy's long-standing crush on Nathan Fillion in Big Mouth may have been resolved in Season 5, but the guest appearance from the actor -- playing not just himself but a body-swapped Missy -- in Season 6's finale brought true closure to one of the show's most character-driven running gags.
How Big Mouth Has Used Nathan Fillion
One of the establishing elements of Missy Foreman-Greenwald's in Big Mouth's first season was her massive crush on Castle star Fillion. Season 1's "Sleepover: A Harrowing Ordeal of Emotional Brutality" revealed some of Missy's fantasies about the actor, even sharing a kiss with the imaginary version of Fillion. As Missy grew and embraced her attraction to boys, her crush on Fillion lingered -- with an imagined Fillion playing a part in her segment from Season 3's "The Planned Parenthood Show" and appearing as a central character in Missy and Jay's erotic fan-fiction in Season 3's "Obsessed."
In each of these appearances, Fillion played himself -- a self-aware and self-conscious version of the actor who was the voice of reason in Missy's head. As Missy became more independent and assertive in Season 5, she "broke up" with the fantasy of Fillion, but Fillion returned in the final episode of Season 6. Instead of just being someone within Missy's head, the in-universe Fillion showed up for a central role in the conclusion to Missy's season-long storyline.
Season 6's finale "F**ked Up Friday" centered around Andrew, Nick, Jessi, Jay and Missy trading bodies with someone in their orbit. While the latter three found themselves in the bodies of family members and Jay effectively possessed a still-conscious Coach Steve, Missy found herself trading bodies with Fillion. Waking up on the set of The Rookie, Missy raced back home in Fillion's body. Meanwhile, Fillion took over Missy's body and adjusted to the wild circumstances that's befallen him. With Fillion's help, Missy opened up to her love interest for the season Elijah and began a romance with him.
Why Fillion's Guest Spot In Big Mouth Works
The Missy storyline ended with the young woman earnestly thanking Fillion for his help, and Fillion admitted it was the "role of a lifetime," before leaving on a plug for his television series. It was a consistently funny guest appearance from Fillion, whose earnest portrayal of Missy was perfectly countered by Missy's voice actress Ayo Edebiri, who played Fillion as something of a cheesy and confident horn-dog who couldn't stop complimenting Missy's mother. It was a solid comedic performance from both performers, but it also completed some of Missy's character growth in a sweet manner.
Missy's overarching arc across Big Mouth focused on her budding sexuality and attempts to grow from an innocent and sheltered child into a more confident and well-rounded young woman. Her childish crush on Fillion had been an ideal vehicle for that development, highlighting how her idealized form of romance didn't add up to a realistic consideration of attraction and sexuality. Having the actual Fillion help her confront a mature decision about her feelings for Elijah allowed Missy to reconcile her hormone-driven desires with her emotional growth, allowing her former crush to help her step forward into what could easily become the show's healthiest romance. It was a sweet conclusion to Fillion's guest spots on the show, while still being pretty hilarious.