The production crew made sure Abigail Breslin really was listening to music in her headphones to keep her from hearing Alan Arkin's profanity-laced scenes.
All of the girls acting as participants in the "Little Miss Sunshine" beauty pageant, except Abigail Breslin, were veterans of real beauty pageants. They wore the same costumes, including hair and makeup, and performed the same talent routines as they had in their real-life pageants.
Steve Carell, at the time he was cast for Little Miss Sunshine (2006), was a relative unknown in Hollywood. According to an article in Entertainment Weekly, the producers of the film worried that he wasn't a big enough star and didn't have much acting experience. However, between the time the film was shot in the summer of 2005 and its release in the summer of 2006, Carell became a huge success as the star of the high-grossing film 40 ans et encore puceau (2005) in August 2005 and the leading character of the popular NBC Emmy-winning television series The Office (2005), which premiered in March 2005 and for which Carell won a Golden Globe in 2006 for best lead actor in a comedy television series. In the span of just one year, Carell had become such a star that the producers had gone from protesting his casting to tapping him to do prominent promotion for the film.
The "heroin" Grandpa snorts in the bathroom in the beginning of the movie is actually crushed up Vitamin B tablets.