Invariably, something breaks, something is forgotten, something goes wrong. The backup is needed. I have always felt distinctly uncomfortable calling someone out of the blue and asking them for an interview. I always feared they would react angrily, shout that their thoughts were not of my concern, and slam the phone down. This has never happened.
People are generally very kind.
Don't be afraid to pick up the phone and call. Most people are eager to talk. Everybody has a story to tell. Explain the assignment. Tell the person exactly what you're interested in, and why.