But for readers (and publishers) it is the characters that take center stage. Knowing them is what captures the reader's interest and so that is where most sources on writing tell you to start.
So let's look at what knowing your characters means. In a nutshell, you need to know what motivates them. What are their goals, hopes, dreams, ambitions? What are they afraid of? What do they like to do and what don't they enjoy?
Equally important is how they express themselves? How do they show anger, fear, doubt, confusion, or suspicion? Note that by 'express', I don't mean just their dialogue. Facial expressions and body language are equally important.
Frustration is another key element. How they react to it controls how your plot unfolds. For example, character A is frustrated that the guild will not hire him. What does he do? He could work harder to convince them. Or he could practice his craft without their approval (risky and dangerous; if everyone did that why have a guild?) Or he could move on into another part of the realm and see if another guild will hire him. Each of these decisions changes the story. The choice you select depends on what makes sense for the character and the story you are trying to tell.
Keep in mind also that a character can react to an emotion in different ways and still be consistent. It depends on the situation. If setbacks make the character angry and want to lash out, perhaps in one situation he or she is warned against doing this. Knowing this ahead of time permits the character to temper the reaction expressed.
So experiment with your characters. Write small scenes that show your characters reacting to different situations. Use these efforts as exercises to learn about your characters. Doing so will help you write them in a more realistic way when it comes time to put together your story.