The most important thing to note here is that this is yet another way that Deadly Premonition accomplishes things that couldn't work in any other medium: letting the player become a co-author in the story. It manages this by having far more content than the story requires. Films have a proscribed length that they can't violate, limiting the amount of story they can feature. Even in the most digressive novel the author must decides the order in which the characters are introduced, and how much coverage they receive—editorial decisions that determine how the audience will react.
While this is true of Deadly Premonition as well, it's true to a far lesser extent—there's so much content to be seen and experienced that players get to decide just how deep an experience they want—and they have to want the experience. This isn't a traditional video game experience where characters talk to NPCs to hear that person's life story and obtain a quest, then get rewarded at the end of it with an item and a little closure for the story they heard at the start. Sallie Graham will never have useful information to offer about her daughter's murder. The player can watch her wail and gnash about the death of her daughter, they can learn her backstory, and follow her to SWERY '65 where she drinks for hours before retiring to Richard's trailer, looking for some solace in the arms of her first love. There will be no reward for any of these activities, beyond a deeper understanding of just how well-conceived Greenvale and all of its inhabitants are.