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Olivia Olson clearly loves the character she plays, because she has Marceline's iconic Ax Bass tattooed right on her wrist. It's the kind of detail that makes you realize just how much some of these voice actors genuinely bonded with their roles over the years.
There's a nice bit of real-life family casting hidden in the show. Olivia Olson's adoptive father, Martin Olson, who was also a writer on Phineas and Ferb, provides the voice of Hunson Abadeer, Marceline's demon dad and Lord of the Nightosphere.
Tom Kenny, who voices Ice King, actually met Olivia Olson when she was just a baby, since he used to perform stand-up comedy at her father Martin Olson's club. Years later they'd end up sharing scenes together as two of the show's most beloved characters.
There's a blink-and-you'll-miss-it moment where Ice King's computer screen shows off his actual browser history, and it's full of pathetic, oddly specific searches about handling princesses and strange health questions. It's a tiny detail but it adds a whole extra layer of sadness to how lonely the guy really is underneath the jokes.
Pause the right frame during "High Strangeness" in season 8 and you'll catch a background world population chart that shows just how brutal the Mushroom War really was, with a population crash on the scale of a full world war. There's even a tiny dip earlier on the chart referencing the Black Plague, which is a wild amount of detail for something most people will never notice.
Way back in an early season, a throwaway line identifies Gunter as the most evil thing certain characters had ever encountered, and at the time it plays as a silly joke. Turns out it was quietly setting up one of the show's biggest cosmic reveals, since Gunter is secretly the ancient being Orgalorg.
Finn's green backpack occasionally shows a little bit of personality and even interacts with him, which is a subtle wink at Dora the Explorer's talking backpack. It's a small joke, but once you notice it you can't unsee it.
The Banana Guards protecting the Candy Kingdom share a suspicious amount of DNA with the Minions from Despicable Me, both in their yellow, rounded shape and their goofy group energy. It's never called out directly in the show, but the resemblance is hard to miss once you're looking for it.
Pendleton Ward only ran the show as showrunner for its first couple of seasons before handing the reins over to Adam Muto. He's talked about how the pressure of running a hit series basically overwhelmed him, and stepping back let him stay involved as a producer without burning out completely.
For a while, Pendleton Ward flat out told fans that Tree Trunks was definitely dead. Then the episode "Crystals Have Power" rolled around and quietly walked that back, bringing her into play again like nothing ever happened. Ice King's age got the same treatment, jumping around over the years depending on what the story needed.
