I don’t know how many times I’ve started to sketch when another person interrupted the line of sight between my subject and me. Rather than start again, I usually try to draw the second subject, overlapping with the original one. This is harder than it looks; contour lines of the background figure are a source of confusion.

Sketch of Man Interrupted by Another Man
Pen and Ink Sketch
10″x8″ 111# Moleskin Journal