- The Cast Showoff: John C. Reilly, already an established singer, is the only actor in the movie who wasn't dubbed and the movie clearly enjoys showing off his pipes. He's also noticeably the only one who doesn't flub playing his instrument.
- Dawson Casting: Parodied, given the trend of several biopics (especially musical ones) where this happens, such as John Goodman playing a teenage Babe Ruth in The Babe, Dennis Quaid playing an early 20s Jerry Lee Lewis in Great Balls of Fire!, 33-year-old Gary Busey in The Buddy Holly Story, wherein he plays Buddy Holly from 19 to 22, and Kevin Spacey playing Bobby Darin throughout his adult life in Beyond the Sea. To put the last one in perspective, Spacey was older than Darin was when he died when he took the part.
- John C. Reilly, aged 42 at the time, plays Dewey as a fourteen-year-old and keeps mentioning his age just to underscore the ridiculousness.
- To say nothing of Edith, played by 34-year-old Kristen Wiig!Edith: I'm his girlfriend! I'm his 12-year-old girlfriend!
- 59-year-old Cheryl Tiegs plays herself in her early 30s as Dewey Cox's wife. Still, it can be argued that she pulls it off fairly well.
- Likewise, we've also got the similarly-aged Patrick Duffy (58) also playing himself in his early 30s.
- Executive Meddling: Sony cut over twenty minutes (including an entire subplot that was shown in the trailers) from the film before it opened. They also released it in a crowded Christmas season, letting it die against the also music themed Sweeney Todd: The Demon Barber of Fleet Street and Alvin and the Chipmunks.
- Non-Singing Voice:
- As Jenna Fischer has admitted, she can't sing. Angela Correa dubbed over the songs for the Darlene Madison character.
- Similarly, the actors playing Dewey's band had to learn how to play their instruments to match the playback.
- Stunt Casting: Parodied - many of Dewey's buddies are themselves (real life) famous musicians... who are often played by (intentionally) ludicrously miscast famous actors (such as Frankie Muniz as Buddy Holly and Jack Black as Paul McCartney), or famous musicians being played by other actual musicians... who are absolutely at the other side of the musical style (such as Elvis Presley played by Jack White). Similar to the Dawson Casting example, they intentionally keep referring to themselves by name to underscore the absurdity.Dewey: What do you think, George Harrison, of the Beatles?George Harrison: It's so dark in this tent, you know, it reminds me of when we, the Beatles, the four Beatles—Paul McCartney: From Liverpool.John Lennon: Oh, we are from Liverpool.
Trivia / Walk Hard