Notes almost always form arpeggiated chords, barring the odd passing note or elision. That's why I stated CHORDS, if you cannot pick the chord runs out from a melody you'll never be able to accompany it. There can be a choice, sure, but ultimately the chords buried in the tune teach you how it will resolve and therefore how to accompany it. The choices allow you to vary the accompaniment and can lead you to modulations to take it somewhere else en route, although for most tunes you don't have time for such fancy stuff.