A fill is a short solo. The fill is typically unwritten (ad lib) and is used as a background effect to add interest to passages in between phrases of the melody of the composition or when the melody is holding long notes. The fill is normally given to an instrument in rhythm section of the ensemble (bass, drum, guitar, or piano), but other instruments can provide melodic fills. In general, a fill is anything that is performed in addition to the written composition to “fill” a silence. It can also underscore a rhythm played by other instruments, announce the entrance or punctuate the exit of a soloist or other passages of the composition, stimulate the other performers to make the performance more interesting. The fill can either maintain the character of the composition or provide a contrasting character. In popular music, a fill is a short musical passage which helps to sustain the listener’s attention during a break between the phrases of a melody.
The bass fill will generally include more aggressive or complex rhythmic figures for the duration required by the fill and can add a more dense harmonic color. The bass, piano or guitar fill can also provide a very subtle and simple melodic or harmonic figure to provide a possible contrast to a loud shout chorus or just to keep in character with the rest of the composition.
Fills can vary as to style, length, and dynamics. Most fills are simple in structure and short in duration.