Quoted in The Beatles Anthology ↑

A gradual development by the boys

I think there was a gradual development by the boys, as they tried to make life a bit more interesting on record. They felt, “We don’t have to go up onstage and do this; we can do it just for ourselves, and for the studio.” So it became a different kind of art form — like making a film rather than a live performance. That affected their thinking and their writing, and it affected the way I put it together, too.

George Martin, as quoted in The Beatles Anthology