There’s a book that I recommend over and over called SAVE THE CAT by Blake Snyder. One of the central ideas is that you can never start building character sympathy too early. And you can’t do it by telling, either, or sharing what the character thinks about himself, or even what other characters think about him. Two of the biggest vehicles for showing (read my perennial post on “Show, don’t tell” here) are choices and actions.
To create a character who the reader will relate to, even if it’s an unreliable narrator, unlikeable protagonist, secondary character or villain, put them in the situation to choose or act as early and as often as possible. This opens up a whole world of potential for you. Do they say one thing and do another? Do they want one thing but choose a path away from getting it? Are they always consistent with thought, speech, and action? All of these things teach the reader about your characters.
Choice and action are very powerful because they show about character, but they also move the plot forward. While it’s possible to take a choice or action back, most will have ramifications. The best choices and actions will be clear dividing lines between a “before” and “after” in your story, whether it’s with a plot, a relationship, a feeling, your character’s self-knowledge, etc. The bigger the choice or action, the more significant it will seem to the reader.
For example, your character is a princess who threatens to run away all the time to escape her responsibilities. Rather than talking about it, or holding it over the heads of those around her (the more often a threat is made without follow-through, the less effect it has over time, per the Law of Diminishing Returns), get her to a place where she has to choose/act. What does it tell us about her if she runs away? What does it tell us about her if she stays?
A type of plot I’ve run into a lot recently has been the “hands tied” or “crash test dummy.” These are passive plots in which the character either can’t do anything because of their circumstances, or gets dragged through the plot by fellow characters or circumstances without contributing much. If your character is in jail, they obviously can’t really choose or act much. That’s a very difficult situation to render in an effective way. Their choices and actions will most likely deal with their inner life (choices reflecting who they are) and relationships (if there are any to be had in the dungeon). At a certain point, though, if your character can’t make any choices or take any action, you need to look at your premise as a whole and decide, honestly, if maybe it’s too limiting to create the sort of dynamic fiction today’s market demands. Sometimes writers back themselves into a corner with a story that’s self-limiting. A “crash test dummy” plot has the opportunity for choice, but the character doesn’t take a stand or act with agency, for whatever reason. It may run into some of the same problems as the “hands tied” type of story unless the character can begin to take the wheel.
Think about whether your character can be described as active or passive. How much do they move the story forward through their will and actions? What plot points has your character spearheaded? Can you call much of what they do or say binding or consequential? If not, you may be underestimating these very powerful tools in crafting character and plot.