In the sequel series Stupid Mario World Rich Alverez plays not 1 but 3 different versions of Mario, the regular Stupid Mario Bros/World Mario, The Plumber Knight Returns Mario and a new incarnation of Mario who was raised in a Bad Future.
The Cast Showoff: The Johnny Mushroom episodes for the characters in the band, and episode 45 for Wario and Waluigi.
Creative Differences: The reason given for Dane Kevin Cook (cinematographer and Ash Ketchum) leaving after Operation Blind Storm.
Old Shame: Kevin didn't like most of the stuff after the movie, and he especially hated Season 5. In addition, he felt that the final bit at the end of the movie's second Act where the Earkness is revealed to still be alive and has one last fight with Wario was anticlimactic, tacked on, and raised a few plot holes.
The Other Darrin: Donkey Kong has had many people in his costume. Even the costume itself has changed in Season 4 (as they didn't actually own the original suit).
Bowser is replaced for Season 5 by Austin (Brock).
Link was originally played by someone else (the voice at the end of Season 1) but he was unavailable for Season 2.
Christian (Ryu) could've played Kid Icarus in Season 2
Julian (Snake) was a big fan of Resident Evil 4 and wanted to be Leon in the show.
Dane was given the option of playing Ness or Ash. He chose Ash cause he loved Pokémon.
Richard (Mario and co-creator) during Season 1 almost wrote several episodes that took place before Mario's very first trip to the Mushroom Kingdom, where he was a normal teenager working part time at a plumbing company, and where Merlin opens the portal to the Mushroom Kingdom and gives him his hat. However he and Chris (Luigi and co-creator) decided that is was too much backstory for a comedy show.
Kevin (Yoshi) had a different idea in mind for Season 5. Scott Masterson, all the way from Season 2's JMK episode, would lead an army of sprites in sieging the Mario Bros.' house. This was overturned. Considering how much he hatedSeason 4, he was seriously considering leaving, but was convinced to stay. In his History of Stupid Mario Bros., he regreted this decision, feeling as though Season 5 was even worse than Season 4.
Word of Saint Paul: Kevin Chamberlain (Yoshi) owns a youtube account here where he often does a series called History of Stupid Mario Brothers (inspired by Linkara'sHistory of Power Rangers) which shows his own opinion on the show and confirms several things in real life that Richie hasn't.
The creators and actors tend to go on the Fan Forums to answer fan's questions. A fan should be warned they don't go on as much as they used to, although Season 4 has already contained characters suggested by fans and fans had bombarded them with villains for Season 5 (though apparently none of them made it in). With Season 5 over they've been a bit more active reading fan's reactions and input.