I found this invaluable!
I first listed all my characters across the top of an Excel spreadsheet and then again down the side. In each intersecting cell I would brainstorm about the relationship (or encounter) a certain character might have with another and put it into that cell. Some cells are blank for cameo roles, and blackened where a character intersects with himself/herself.
This method forces an author to flesh out my plot further and create a rich tapestry of character interaction. In the real world, characters seldom interact with just one other character around them. Friends meet friends’ friends, and interact – and so on.
This technique will keep your work from being one dimensional and allow you to create sweeping epics like that of James Michener, or at least stories with depth.