Their immediate popularity is also related to image. With this album, the Beatles came of age. Affected by acoustic music of the folk revival.
Revolver moves away from conventions of commercial rock or pop album.
Many songs credited to both though often the influence of one or the other clear. McCartney more solid musically and Lennon more so lyrically. By 1966, they were beginning to really write more independently. Shows McCartney's focus on melody, chord structure. Lyrics very straightforward.