For a reader to invest upwards of 80,000 words or more into a character, there must be something worth following. Flat characters spell doom for a book. Even if your novel is a plot-driven mystery, the characters should be as developed as possible. Every character should have a backstory that gives nuance to her actions, or reasons for her choices. Supporting characters deserve the benefit of a personal history too, and for a writer just beginning the process of writing a manuscript, a set of character details can enhance the story even if the reader never necessarily finds out every tidbit that you used to build the character.
It's easy and often tempting to start with plot points, beats within a scene, or crafting dialogue and then dive into writing with a few sketchy character details (he's an innocent man unjustly accused running from the police, etc.) . But the characters' experiences shape their decisions, which translate into conflict. Their personal history affects how they speak, or whether they speak at all in any given situation. Here are some ideas on making your characters more round.
Start with some of the things a person may answer on a dating profile, or in a job interview, or even if they were writing an autobiography. Everyone had parents, foster parents, a father/mother figure, a teacher who influenced him. Who were they? Did they shape the character for better or for worse? Where did your character go to school? Or did she drop out? This affects self esteem and socio economic status, which drives the characters' choices, which in turn, will feed into the novel's conflict. And your book needs conflict. Lot of it.
What's your character's ethnic, racial and religious background? It's a great big world out there and your book should reflect it. What are their spiritual and political beliefs? Not having either is fine because that can also explain how people behave, how they react to the world, and how they treat others. It's easy to have a cookie cutter "stern librarian," or the familiar beleaguered police sergeant who keeps saying he's "too old for this," but dig deeper: Did the librarian have a childhood dream to become an actress that was squashed by an abusive parent? Does the sergeant have a sick spouse at home and is only on the job to keep his insurance benefits? Just picking a character's name, gender and occupation isn't enough. You should know why they have the job they have, who they love and who they lost, and how they view themselves in the world. Do they drink scotch or beer?
Does your character have a secret? Even if the "secret" is as benign as a corporate executive being a Star Trek cosplay nerd, this gives him depth and makes him realistic, and ultimately more interesting. If she has a body buried in the backyard, bonus!
What is your character's dream/goal? If he has an unfulfilled dream, and is stuck in a job he hates, that again gives him curves and edges. It's easy enough to describe a rather overweight person eating fast food. But if you have that character looking at the food in disgust even as he eats it, alone, in his car in the parking lot of McDonalds, your reader knows this is a lonely, sad person who uses food for comfort. You, as the author, already know that his adult kids hate him, he hates his job, and he used to be a college athlete who has let himself go. You don't necessarily put all that into the narrative, but just knowing that will help you craft a few sentences that give him some texture, and that makes him come alive for the reader.
Some enterprising authors make a spread sheet with all of the characters that lists all their relevant information. You don't have to necessarily go that far, but remember that the plot still has to be carried by people, and the reader must want to read about these people dealing with that plot. Rounding out your characters will make the reader stick with your story because you've created people worth caring about.