Bell suggested creating a grid listing the characters in your story along both axes. In each box of the grid, you add possible relationships, secrets, and areas of conflict between the two characters whose names line up there. For example:
|Fred||X||Dave is jealous of Fred for getting the promotion he wanted at work||Jane knows that Fred's wife is having an affair|
|Dave||Fred considers Dave a good friend, but is tired of covering up for his mistakes at the office||X||Jane saw Dave drinking in the office, and watched Fred throw out the bottle when Dave wasn't looking.|
|Jane||Fred thinks Jane is a bit nosy and kind of a gossip, which is something he hates||Dave secretly thinks Jane is very attractive, but hasn't trusted her since he saw her take money from the petty cash box and slip it into her purse.||X|
Brainstorming all of these points of connection and conflict between the characters could improve the complexity and depth of the relationships depicted in the story. For example, if Dave tried to make Fred look bad so that management will demote him, Fred might admit that he's been covering up for Dave. Jane could back Fred's story up, since she's seen him throw out Dave's bottle.
Or, if we wanted to start an affair between Jane and Fred, the fact that Jane knows Fred's wife is fooling around is a secret she might slip to Dave. Dave could use that to get back at Fred in retaliation for the promotion he thinks he deserved. By concealing her knowledge of Fred's wife's affair, Jane could engineer situations to be with Fred and console him, improving her chances to catch him "on the rebound". Of course, if Fred finds out that Jane knew about the affair and didn't tell him, he might want nothing more to do with her.
Michael A. Stackpole describes something very similar in an article on characterization techniques that appears in his The Secrets writing newsletter (Volume 1, Issue 23). He calls this "Interaction Dynamics" and tells us:
Characters do not operate in a vacuum… once you begin to see how they will function, you have to then compare and contrast them to other folks in that world and other characters in your story. In essence this is a playtest of how the characters will function inside their world.
You are looking for affinities and oppositions. Who is this character's natural ally? Who is her enemy? Is she willing to compromise on some point of pride to make thins work with this person? What behavior by someone else would turn her against him, or endear her to him? How will she react if an enemy spares her life or saves it?
…None of us have a relationship with one person that is unaffected by our relationships with everyone else. We just don't operate in isolation like that – or if we do, there is some serious pathology that needs to be dealt with.
…Between and among the major characters… there should be a lot of energy and strength, whether they get along, as passionately in love, or hate each other's guts.What all this boils down to is that our characters aren't robots, they're people. They don't necessarily like (let alone love) everyone else in the story. They may not trust some of the other characters. They may, even subconsciously, want another to fail. All of these "connections" can be used in your story where and when they make sense.