Sustained notes over changing notes in same staff

For a single instrument (piano, guitar) how do you write an upper line that changes with different rhythmic values over sustained notes in the same staff?

For example, in the treble clef, a quarter note with eighth notes above it, or the reverse; half note chords in the bass with quarter notes that "walk" from one chord to the next below it.

This is straight forward if the pitch values are in different clefs, but how do you do it in the same clef?

Thanks,