I've got two theories about the behind-the-scenes of Book 1, one of which is gut feel and the other which has some data to it:
Sidenote: Don't compare Brennan's bar to the players'. In order for you to see any variation in the players, I needed to give Brennan his own color scale.
Protagonists exert a gravitational pull on minor characters, and in WBN, most NPCs orbit just one PC. Each point on this chart shows how evenly a character is shared by the PCs, with Will Gallows being spoken to equally by all three, Ghost being spoken to mostly by Ame and Suvi, etc.
The following characters are only ever addressed by one of the PCs: