Rex Harrison was scared to death of going into a sound studio to record his vocals. He was allowed to sing "live" directly onto the soundtrack, as the film was made. Incidently, Alan Jay Lerner wrote the lyrics specifically to be half sung/half spoken. They were written especially for Harrison, as he was the one and only choice for Higgins when they originally wrote the musical.

In the 40th anniversary re-mastered edition they have used Audrey Hepburn's original singing track for the "Wouldn't it be lovely" song.

Rex Harrisson also played Henry Higgins on broadway. He wanted Julie Andrews for Eliza as well, since she had also played Eliza on broadway. (Julie didn't get the role obviously, but did play Mary Poppins that year, and won the Best Actress Oscar).

Audrey Hepburn was not the only voice that was dubbed. The actor who plays Freddy had his voice dubbed as well, so in their scene together with the song 'show me' neither of them are singing.

Cary Grant was offered the role of Henry Higgens by Warner Brothers. He turned it down and said that if any one other then Rex Harrison, the actor who was Higgens on the stage, got the part he would never do another film for WB.