In later interviews about the film, Steve Allen admitted that the real Benny Goodman (who he had met during the making of the film) was not nearly as pleasant an individual as Allen had portrayed him in the movie.
Benny Goodman played all the clarinet music heard on the soundtrack himself, with the exception of the scene where the young Goodman first tried to play the clarinet - the squeaky notes were actually played by Steve Allen.
Although Ziggy Elman plays himself in the film and is seen performing his famous trumpet solo in "And The Angels Sing," Elman was actually only playing "air trumpet." By that point he was unable to play (he eventually sold used cars to make ends meet and died in relative poverty) and so the solo was performed on the soundtrack by another trumpeter, Manny Klein.
The screenplay was heavily fictionalized, but the music was real. Many of Goodman's professional colleagues appear in the film, including Ben Pollack, Gene Krupa, Lionel Hampton and Harry James.