It can be hard to get inside your character's head. Sometimes the plot idea comes first and the main character seems a bit fuzzy, or you can't quite decide who they'll be. And because character backstory, motivations and actions/decisions all feed into the plot, if you go ahead and develop your plot without doing the work on your character, you end up with someone who doesn't feel "real". Her/his voice is bland, actions seem inconsistent, motivations unclear - all this leads to the reader not being fully engaged with the story.
It's almost a rule in fiction - if the reader doesn't care what happens to the main character, they won't want to read the book. I say almost a rule because characters like James Bond work differently, but mostly your main character needs to be strong and engaging.
Then comes point of view. Beyond whether you tell your story in first person or third person (intimate/subjective - there are different terms for this), if you are using that close POV, the reader wants to feel they are right there with the character, thinking their thoughts, feeling their emotions, experiencing their life with its highs and lows.
Sometimes this inner bonding with your character comes easily, maybe because they are part of you, or express a part of you that you explore via the story. You hear their voice, seem to know them intimately before you've written more than a dozen pages, understand their strengths and failings and how these will figure in your story.
But more often, you have to make them up, and then make them real. Masha Hamilton (at the Pima Writers' Workshop) said that probably only 10% of what you know about your character will appear on the page. The rest is all the stuff you need to know and understand about them, their life, their backstory, in order to be able to write about them convincingly.
There is a trap within first person narrative. It's the assumption that once you "get" the voice of that narrator, the rest will fall into place. If you are able to write your character onto the page as you go along, great. But you have to do a lot of writing to get there.
It's easier to do the background stuff first, even if it looks like a lot of extra work.
I see a lot of students struggling with POV, and ending up with pieces of writing or chapters where everything seems to happen at a distance, as if the narrator is just reporting, or it's the author who is doing all the work and the narrator is looking over her/his shoulder. A simple sign that this is happening is when the narrator refers to their mother as "my Mum" (as in - My Mum said, "Where are you going?"). When you are right in the POV, your narrator would usually just say Mum - Mum said, "Where are you going?".
This is where I am right now. I am up to Rewrite Number 8 on a manuscript that I haven't looked at in nearly a year, and I've kind of lost my grip on who my characters are, why they're doing what they do in the story. I also need to do a lot more work on my other major characters, to avoid them being one-dimensional and poorly motivated.
My mantra is: Every character has their own journey in the story.
And I need to know what it is. Out with the notebook and pen, and away I go.