Mental Floss's Rob Lammle has researched an admirably thorough history of The Animaniacs, one of my all-time favorite cartoons.
Of the Warners, the voice of Wakko was the most difficult to cast. During auditions, the producers said they were looking for "wacky," so all the actors delivered a voice that was over-the-top crazy, but none were the right fit. On the last day of auditions, Ruegger brought his 1990 Almanac to the office, hoping to find some inspiration that might shake things up. Many wacky Wakko's later, they still didn't have the right voice. So during their last appointment of the morning, with voice actor Jess Harnell, Ruegger opened the almanac to a list of celebrities and asked Harnell to do his best impression of Elvis, Rodney Dangerfield, Jackie Gleason, Frank Sinatra, and other notable names. When the Beatles came up, Harnell proceeded to do every one of the Fab Four so well you could actually tell which individual band member he was mimicking at the moment. However, it was Harnell's Ringo that struck a chord with the producers, so after a few tweaks, that became the voice of Wakko.
To promote Animaniacs before the show's premiere, a giant balloon in the shape of Yakko was placed on top of the water tower on the Warner Bros. lot. Unfortunately, no one told Bob Daley, who ran the studio. When he pulled into work that morning, he thought someone had put a bad Mickey Mouse balloon on the tower and ordered it removed. The inflatable Yakko was in place for less than 12 hours, and then popped shortly after he came down. Writer Paul Rugg was able to snap a photo to prove it happened.
After the balloon incident, Daley worked to ensure no one else would mistake the Warners for Mickey. Daley decided that Yakko and Wakko were too smooth and rounded. So while he watched, he had Ruegger add side whiskers to the drawings, which he felt would prevent confusion – and potential legal action. Ruegger and Warner Bros. Animation president Jean MacCurdy had to rush back to the animation studio with the changes, because the cartoon was already being drawn, with some segments in the can. You can see the Before Whiskers and After Whiskers comparison below: