Even if you do not know what actions your protagonist will take in your story, or even what lies ahead in upcoming chapters, you need to know your protagonist like he or she (or it, depending on what you're writing) is your best friend and soul mate. This can be exceptionally tricky if you've never worked with this character before and are developing them from scratch. However, this gives you a blank canvas - a chance to get to know them from the ground up and turn them into a fully developed entity.
There is nothing worse to a reader than an undefined, bland, and under-developed character that they are made to follow throughout a story. You are giving them an intimate look into this character's life, and they need to be able to feel for your character and root for them. It's hard to root for someone that you can find no emotional connection with. That emotional connection is created by you, through your handling of the character's physical attributes, personality, and lifestyle choices.
A name is usually the first thing that comes when creating a new protagonist. With that name comes a gender. Name your character, and then introduce yourself to them. Make friends with them, as silly as that might sound. Yet, it shouldn't be silly, as they don't only live inside the pages you're typing. They live inside of you as well.
How old is your protagonist? Are they a child? A middle-aged man or woman? An elderly person with a great backstory? You need a general idea of this because your character, like real people, develops as they age. Once you've determined the name, sex, and age of your protagonist, it's time to develop who they are - who the world will see when they read your book.
Pretend like you're on a blind date with your protagonist, or better yet, pretend like you're a journalist. Your protagonist is the subject of an interview assignment that you've been given, and you've never met this person before and have little to go on for your list of questions. So, it's time to make that list of questions to ask them. Things you need to know to understand your protagonist include where they are from, what their early family-life was like, did they earn a higher education, how do they take their coffee, do they have any food allergies, and what colors best compliment their complexion, eyes and hair. Do they have a favorite movie, book, or song? What makes them happy? It could be anything from a nice rain on a spring afternoon to roller skating in the park.
Does your protagonist have a job? What do they do, and how did they get that job? Are they close to their colleagues? Do they have a family at home? Are they married? If so, how did they meet their spouse, and what was the attraction? Are they looking for love? What keeps them from finding it? Do they have a sense of humor, or are they edgy and serious? If today was their birthday, and you've known your protagonist only for a few days, what would you buy for them as a present, using your current real life budget?
As you write, your protagonist will continue to throw surprises at you, but as long as you have a good sense of who they are, you'll be able to roll with the punches. Things will happen. Maybe dark secrets from the past will turn up - out of the blue. Perhaps your protagonist will suffer a personality change, becoming angered or devastated over some happening or even something not happening. You must know your protagonist enough to be able to handle them properly when these things occur. Your character cannot behave in one fashion for the first half of the book, and then suddenly act like a completely different person through the rest. There must be a reason for such a change in personality, and it must be justified by things that have happened to them currently and in their past.
You also need to be able to trust them to confide in you. If they've had break-ups that were bad, you need to know early on so that you're not blindsided. If they have a dark past or a deep want or need that will be exposed later, you need to be able to justify that as well. Make a list of everything you can possibly come up with about your protagonist's past and current life situations. If they smoke, how often? If they've been dumped, why? If they've been fired, for what? Are they shy? Why are they shy? What happened to make them shy? Was it something from their childhood? Perhaps they were ignored when they were little. Maybe the other children in class teased them because they had big ears or walked awkwardly. When a reader is reading your book, they need to see your protagonist as a real person, even if you're writing fantasy or science fiction. If your protagonist is a robot, you need to know who made it, why, when, and out of what. The protagonist is one of two characters in your book that you cannot take shortcuts on or find an easy way around. The other such character is your antagonist, which we'll discuss another time.
Even if you don't use it in your book, now that you've interviewed your protagonist and made a list of their likes, dislikes, victories and failures, write their backstory as a story. Get it all out, right there in black and white. This will help you further develop the character so that, when you go to write your book, you have a full and complete understand of where they are coming from. You'll be able to tackle those issues of personality changes or varying emotional states because you've prepared. You've gotten to know your character's history, and so you can move forward while still being able to keep them true to themselves. Even in horror, when a protagonist is forced to kill the bad guy, there are reasons they are doing it, there are emotions they are feeling over it, and there are circumstances in their past that have given them the strength to survive. All of this makes that protagonist who they are in that moment, and it should be visible through your words - the actions and feelings you deliver in that scene and in the scenes leading up to it.
Remember, no one's life is perfect - in the real world or fiction. Everyone faces pain, anger, suffering, loss, and heartache at some point or another, and your characters should be just as flawed as real people. It's hard to root for perfection, as perfection is something that can never be achieved. As long as you know who your protagonist is, you can offer up a realistic character that can make the reader feel what they are feeling, like what they like, hate what they hate, and love like they love - right along with them. That is the key to a well-defined and well-rounded protagonist, and in order to achieve it, you must know your character.
In my early days of writing, I was guilty of offering one-dimensional characters - driven only forward with no realistic reasons and no true identity. This is something to avoid. Like my characters, yours will become more defined and more realistic the longer that you write. I'm an avid fan of Archie Comics, and even sweet Betty has a mean streak and vain Veronica has a soft side. Your protagonist must have both the yen and the yang - a bit of good and a bit of fire - to come across as real to your reader.
So, you've interviewed your protagonist, you've made your list of traits and events, and you've written their backstory. Where do you go from here? Have you created an emotional bond with them? Do you hear them in your mind, aching for their story to be told? If so, it's time to open up your new document and type the words 'Chapter One.' Good luck!