Know your character well before you start to write. Spend some time with them, have conversations with them that don’t relate to the book. See them in scenes not in the book. Interview them.
Refrain from constant use of POV character’s name unless clarity is needed.
During first revision, take note of what we notice first about the character. Is it consistent throughout book? What do we like about the character? Count how many things we learn about the character in the scene where we first meet them. There should be several, and most of those should be internal things, such as conflicts, preferences, attitudes, etc.
Think about who your character loves/cares about, and who loves/cares about them, even your villain.
Give them people in their lives: family, friends, children, etc. They give your characters human ties and vulnerabilities.
Think about each character’s two biggest actions in the book. Are they properly motivated? Foreshadowed?
Their quirk should be evident in every scene, at least once
Look for all opportunities to show characterization. For example, if the heroine has sneaked into someone’s office and is using a piece of paper from the trash to take notes, make that piece of paper something relevant to the office owner’s personality. What could it be a receipt for?
Do they expect certain behavior from secondary characters (and each other) based on their past? Do they ell stories, make references to the past?
Do they react appropriately? They should always react to everything that happens.
Are there cultural and class differences between hero and heroine that you can explore/exploit?
Are you characterizing using action and not description?
Do you show enough of their jobs/profession to make it believable?
—professions/hobbies color our speech, esp modifiers
And most importantly, are these the kind of people you’d like to spend a few days with? Because that’s what your reader will be doing.