Woody Allen had to tell Goldie Hawn to try to sing worse because she sang too well to be believable as a normal person just breaking into song.
Woody Allen did not tell the actors that it was a musical until after they signed their contracts. None of the actors who appeared in this film had singing experience, and this is the only time any of them had acted in a musical.
The actors sing their own songs except for Drew Barrymore, who convinced Woody Allen that her singing was too awful even for the realistic singing voices he was after. She said she doesn't even sing in the shower.