The day before McCartney wanted to work on the song’s score, he learned that George Martin, who usually handled the Beatles’ string arrangements, was not available. He contacted Mike Leander, who did it in Martin’s place. It was the first time a Beatles song was not arranged by Martin (and the only time it was done with the Beatles’ consent: Phil Spector‘s orchestration of Let It Be was done without McCartney’s knowledge). Martin was hurt by McCartney’s actions, but he produced the song and conducted the string section. The harp was played by Sheila Bromberg, the first female musician to appear on a Beatles record.
The stereo version of the song runs at a slower speed than the mono mix, and consequently is a semitone lower in pitch. This is mentioned in the booklet accompanying The Beatles in Mono CD box set, but no reason is given. A 2007 Mojo magazine article revealed the mono mix was sped up to make Paul sound younger and tighten the track.