Right phonetic
分 — supplies the sound (fēn → bàn, labial-shift drift; same phonetic series gives
份 fèn,
盼 pàn,
纷 fēn). Originally a knife (
刀) splitting (
八) something. In
扮 the dividing image fits: dressing up assigns each person their part, dividing roles among the cast (hence
扮演).