“John Lennon wrote this song, which may have been influenced by the ambivalence he felt during his first marriage,” (Songfacts). “Lennon called this song ‘my first attempt at a ballad… it’s semi-autobiographical, but not consciously.’ Lennon and McCartney sang together into the same microphone when recording this song. John sang the lead on the intro, then Paul sang in a higher lead while John sang harmony.” The tune “was used as the B-side of ‘And I Love Her.’
Paul McCartney said of his songwriting partner: ‘People tend to forget that John wrote some pretty nice ballads. People tend to think of him as an acerbic wit and aggressive and abrasive, but he did have a very warm side to him really, which he didn’t like to show too much in case he got rejected. We wrote ‘If I Fell’ together but with the emphasis on John because he sang it. It was a nice harmony number, very much a ballad,'” (BeatlesBible.com).
The intro of the 1964 track, a single from the Hard Days Night album, is largely in C major, but transitions to to C# major at 0:34, where it stays for the balance of the tune.