There are many writers who ignore taking the time to really get to know their characters intimately. For characters to stand out, you need to do an in-depth character background, on each character, so you know them inside and out, including their detailed background, flaws and strengths.
Know your character’s morals, upbringing, and education and ‘core’ values are essential to writing their dialogue and reactions. You don’t need to worry if these details make to the scene and page, but as a writer you do need to know who they are and how they would ‘react’ in most situations. The most important element is to include the character ‘flaw’ which makes them stand out ‘emotionally’ to the audience. Once they get what’s troubling them and can relate and start to route for them. It is always the ‘human flaw’ that the audience remembers the most about the characters. And how they overcome those ‘flaws’ in the end.