Consequentially

All relevant dramatic activity is rooted in consequences, which in turn is linked to event.  Even in the opening scenes, characters appear as a consequence of backstory activity and planning.  Consequence is the result--singular or plural--of behavior or its specific absence; it is the metaphoric rock being dropped into a pond, the ripples being broadcast upward, outward, downward.  Or, of course, a conspicuous rock that is hefted, then not dropped into the pond.  We may be just as pleased or disturbed by an event not taking place as we are affected by its performance.

Consequence begets more activity--individuals being motivated by or responding to or hit by yet another metaphor--falling bricks.  These have been dislodged by previous activity or its lack, an effective illustration of how story runs on the fuel or energy provided by the introduction of consequence.

The most minor characters and their most nuanced front-rank counterparts define themselves by the manner in which they act or withdraw from action, setting forth the ripples, helping us anticipate the consequences in terms of how other characters will respond to what they have said, done or dithered at.

The consequences of characters doing nothing, neither reacting nor initiating some form of gambit in search of a winning agenda, are in delightful irony transferred to the reader, whose revenge is the act of setting the book down or, as Dorothy Parker put it, "This is not a book to be set aside lightly; it should be hurled across the room with great force."  The characters themselves may be propped up by the author via endless philosophical conversations, by lavish smorgasbords of description, or narrative disquisitions of backstory, but without the lurk and menace and promise for misadventure of consequences, they will fall as though they were another metaphor yet, the house of cards.

In order to make readers care about characters, you need to make the consequences of their behavior nudge them into the approaching traffic of vulnerability, where every step becomes a hazard.