Among the main and secondary cast, Grey DeLisle plays Frankie, Goo, and Duchess while Candi Milo is Coco, Madam Foster, and Cheese.
The show very rarely had guest stars - if a new character shows up in an episode, it's going to be voiced by somebody who already voices one of the main characters (and if it isn't, it'll usually be either Jeff Bennett or Kevin Michael Richardson). It's pretty obvious how many characters in the show in particular are voiced by Tom Kenny or Grey DeLisle.
The Character Ice Cream Bar: Blue Bunny released an ice of Bloo with black gumball eyes. Due to his simple design, it's literally his entire body.
Completely Different Title: In Spain and France, the series' title was oddly translated as "Foster, the House of the Imaginary Friends", leaving "Foster" in English as if it was the place's proper name instead of part of a noun.
Creator Backlash: Lauren Faust has made it abundantly clear that she is not at all proud for writing "Everyone Knows It's Bendy."
Creator's Favorite Episode: Mac's voice actor Sean Marquette cites "Partying is Such Sweet Soiree" as his favorite episode, specifying that he really enjoyed getting to perform the role of Mac while his character was running around screaming like a maniac from his sugar high.
Lauren Faust admits that she hated Bendy just as much as the audience did, which is why he never appeared again.
Tara Strong has said that Terrence was her least favorite character to play, as she went hoarse from putting on such a raspy voice (not helping the fact that around the same time she also voiced Dil Pickles from All Grown Up! who also shares the same voice tone). note This is one of the reasons he appeared less in later episodes.
Played straight in the French dub too — Mac's voiced by Nathalie Bienaimé.
Dawson Casting: Eight-year-old Mac is voiced Sean Marquette, who was about fifteen or sixteen when the show began. He was not replaced after he had gone through puberty. Instead, he had his voice digitally pitched up. It is obvious if one compares episodes from Season 1 to episodes afterward.
Descended Creator: Series creator Craig McCracken voices his caricature, a cartoon executive, on "One False Movie".
Development Gag: In "House of Bloo's", Bloo is shown at one point watching a Lassie parody. The boy in the show has the same character design as Mac's prototype appearance.
DVD Commentary: The first season DVD set includes a commentary on "Store Wars", where Frankie and Mac are trying to sign a treaty after Bloo caused trouble in the events of the episode.
A minor example, but this is the reason why "Everyone Knows It's Bendy" ended up being such a disappointment to the writing staff. Before the episode was finished, Cartoon Network insisted that the series switch from half-hour to quarter-hour hour shows and the result was that several key story moments, which would have ended in Bendy and Bloo getting what they respectively deserved, had to be dropped.
For a different example, Craig McCracken talked about how the network wanted the show to remain episodic and rerun the episodes in any order. As such, there was no long lasting character development for anyone, most notably Bloo.
Follow the Leader: While not the first TV series to use Adobe Flash for its animation, it was arguably the first to push its limitations to better suit the looser appearance of traditional character designs instead of the blocky, geometric look often used to make Flash animation easier, and has clearly had an influence on television fully embracing the medium over the following decade in shows like My Little Pony: Friendship Is Magic and Motor City.
Inspiration for the Work: The inspiration for the show came from when Craig McCracken and Lauren Faust adopted three dogs from an animal shelter, and wondered what their lives were like before.
In 2006, Burger King released a set of mix-and-match and create-a-face toys.
In 2007, McDonald's sold a set of eight toys based on this series, Camp Lazlo, My Gym Partner's a Monkey, and Squirrel Boy, with each franchise having two characters. The Foster's toys were a plastic figure of Wilt with a stencil in his back, and a wind-up vibrating pencil topper of Bloo.
No Export for You: The first three seasons are avaliable for people to watch across multiple regions of the world on both physical (DVD) and digital media (Amazon). Seasons 4, 5 and 6 on Amazon US, however, have been restricted to Region 1 only. Unless you had a way to bypass the region IP lockout and include an attached US address to your account, it's impossible to see the conclusion of the series. No explanation has been given for this odd decision.
Google Play and Youtube also have the entire series restricted to Region 1 as well. Again, no reason has been given for this odd and frustrating choice.
Out of Order: The episodes on the first season DVD set are listed differently than how CN aired them, as they go from production order. This makes sense as "Busted" feels like it takes place shortly after "House of Bloo's".
Pop-Culture Urban Legends: Around 2006, there was a Wikipedia article posted about a Big Damn Movie in which Mac and Bloo entered a secret door in the building and ended up in a strange new world. It was confirmed as false (and the article subsequently deleted), but it may have inspired the movie Destination: Imagination, released two years later.
Tribute to Fido: Not only was Craig McCracken inspired to make the show after he and Lauren Faust adopted their dogs and imagined what their lives were like before getting adopted, but they get referenced twice in the show. In "Who Let The Dogs In?", Mac names two of the puppies "Buckaroo" and "Lucy", which were what Craig and Lauren named their dogs. Two dogs modeled after the real Buckaroo and Lucy appear at the beginning of "Bus the Two of Us" in a car the Foster's bus passes.
A character originally conceived for the show was an imaginary friend who was created from the acid trip of a college student. However, Craig McCracken had no idea how to convey it subtly before realizing he didn't want to know what kinds of imaginary friends an adult could create, hence the rule that only children can make imaginary friends.
According to the show's pitch bible, Mac and Terrence's parents divorced shortly before the events of the series.
Write Who You Know: Frankie is based on Craig McCracken's wife and series writer Lauren Faust. He denies that Mac was based on himself, but seeing as Mac has a crush on Frankie, McCracken eventually decided to roll with it. The concept came to him when he and Faust adopted a puppy and McCracken made up a story about what his life was like before they got him.
, Craig McCracken talked about how the network wanted the show to remain episodic and rerun the episodes in any order. As such, there was no long lasting character development for anyone, most notably Bloo.