The key to writing engaging prose is to create compelling characters. Your story comes alive when you have stretched your characters out to the point where they surprise even you. Your storyline and setting may be solid but if your characters are flat, your prose will be flat also. 
Think about it this way: what makes a movie most memorable for you? The characters, isn't it? They are so compelling that you want to see the movie multiple times and may even fantasize about them. This applies to your prose. To keep your story in the mind of your readers while they read and even long after they have finished reading, your characters must present a clear definition of who they are, their fears, values, motivations and choices. How you present your characters will cause your readers to engage with the plot by developing an emotional connection with them.

Depending on the type, your characters may change as the work progresses or remain the same. Either way, it is important to keep an open mind while you write. Allow your character a mind of their own and get to know them as they follow the progress of your story. One way of doing this is to furnish your characters with real-life-like characteristics and elements like hobbies, history, families, daily struggles, obsessions, aspirations, needs, and desires, and then allow them to react naturally to the pressures of these characteristics.
Character development refers to the process of building fully rounded characters who have clear motivations and personalities, and can evolve throughout the story.

How to Develop Your Characters

A well-developed character is a three-dimensional character with clearly defined motivations and personality who changes as the story progresses. As a writer, you must clearly define your character's motivation and establish what the stakes are. If you can do this in a way that is relatable to your readers, you would have successfully held up on your end. 
Below are some pointers on how to develop your characters: 

1. Create a background for your character. You can do this by preparing a character development sheet where you draw up a history for your character; outlining a childhood, family, relationships, interests, events and anything that serves as a pointer for what their lives had been until we meet them in the story. 

Having a background will give your readers perspective. 

2. Create flawed characters. This will help your readers relate with your story better. 

3. Know your characters'  strengths and weaknesses.  This can also go into your character development sheet. The strength and weakness of your character can affect their perception of events and can create conflicts for them. 

4. Create dynamic characters. Your protagonist should change as the story progresses.

5. Create villains with a motivation that readers can sympathize with or understand. 

6. Secondary characters should complement the main characters but exhibit traits and characters of their own.  It is okay for your secondary characters to have views and opinions and manners that oppose those of the main characters. 

7. Your characters should reflect the story's message. Writing a story is a good way to pass across a message. Use your characters setting to buttress the message, for instance, an evil character whose habit is to wipe clean his beautiful china set can be figurative for "cleaning the exteriors while the inside stinks".  

8. Show more than you tell - prosaic works are usually heavy on details. Use descriptive details to reveal the physical world of your characters (and even their interiority through internal monologues, inner conflicts and opinions). When your readers can feel, see and sense the same things as your characters, they'll be able to connect emotionally with your story.

9. Create character traits, and skill sets [for each character] that compliments their role.
Have a wide variety of personality types. No two characters should be similar and give each character a distinct voice. 

10. Avoid stereotypes
Lead your readers on then introduce twists. 

Basic Character Development Questions

1. Who is my character? - their name, age, birthday, physical looks and general disposition. 

2. Who are their families? - Names of people they share a relationship with and type of relationship, relevant relationship history, where they are, what they look like. 

3. How does my character behave? - quirks and features, how they react to anger/ joy/ pain, habits, how they dress, are they introverted or extroverted? what they eat.

4. What are my character's strengths and weaknesses? - where/at what do they excell/fail, and what areas they would like to change?

5. What is my character's history? - school records and academic position(s), how they have changed so far, and what they were like before.

6. What are my character's beliefs? - morality, religion, and their standing on  day-to-day subject matters.

7. What are my character's values? - what they value most, how they react to negative situations, discord and stress. Their life's purpose, source of fulfilment, goals, dreams, wants and ambitions.

8. What are my character's experiences? - notable events in their history, traumatic experiences, childhood, how far they have come, what they currently have going, and what they have built for themselves.

9. What are my character's interests? - hobbies, favourite food, pastimes, first date
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