Executive Meddling: John Lasseter strongly suggested several changes to the original director, Chris Sanders; Sanders refused, causing some considerable behind-the-scene drama. Considering the source, though, the interference was probably justified. As a result, Sanders left Disney and now works for Dreamworks Animation. Go figure. Though the original version did look a lot more distinct with Sanders' art style. At least the original version of Mittens, a cat with an eyepatch named Mercury, got his own Web Comic, Kiskaloo.
"I saw the latest reel (yes, I worked at Disney then) and it was the most disjointed, confusing, quirky, weird, unmarketable movie Ive seen. It was rife with plot problems, the characters were unappealing... it just wasn't working and Sanders wouldn't fix it."
But, then again, others who have seen the original version have stated that Disney staff were afraid that it was "too mature".
The Other Darrin: Another dog takes on Bolt's show role while he is gone; the actor Penny immediately recognizes it's not "her" Bolt, but continued on for the sake of the show.
The Other Marty: Chloe Moretz was the first choice to voice the role of Penny, and apparently voiced the character for the entire film. But she is quickly replaced by Miley Cyrus, who went on to re-record most of Penny's dialogue, even though Moretz's voice for Penny as a child still remains.
Stunt Casting: As mention above, majority or Penny's lines were already recorded before the studio decided to recast Miley Cyrus to voice the character. Recasting to feature their current Teen Starlot in the movie. Not really a coincidence.
What Could Have Been: The final version is far different from Chris Sanders' original American Dog idea. Originally Bolt was more of a Funny Animal secret agent character, Rhino was a giant radioactive rabbit and as mentioned above, Mittens had an eyepatch and was named Ogo. Also... Executive Meddling.
Looks like the writers of Cats & Dogs decided to take that original script.
Also, former NFL player Todd Marinovich was in talks to do the voice of Bolt.