Before you begin to write your book, you pretty much have a good idea about your main characters. You know what they look like, you know about their psychological makeup and you are pretty well-versed in their history and motivations. But you can take it up a notch and develop them even further to make them really stand out.
I am currently reading a novel where the story is ambitious, the plot well-paced and the setting lush but the main characters are a bit of a disappointment. They are very black and white and I am left with a feeling that they are nothing more than exaggerated cut-outs. Unfortunately, it has detracted from what could have been a phenomenal story. All stereotypes are accounted for: the good and loyal heroine, the nasty sister, the doting father, the greedy, evil bad guy, the list goes on and on.
Make them interesting. On paper your main character (MC) might be perfect. But is he/she perfect to the point of being boring? Tweak it a little bit. Perhaps she is beautiful with one brown eye and one green eye (interesting). Perhaps he excels in a competitive, take-no-prisoners type of job Monday through Friday but during his down time, he takes his beloved dog to the local pediatric ward.
Make Us Care
Make them sympathetic. I hear this complaint a lot from agents as well as readers. The refrain goes something like this: I stopped reading the book because I just did not care about the main character or what happened to them. The reader has to care or they are not going to finish it. You make them care by creating a link between the MC and the reader. And you can do this by using any of the universal themes and have your MC experiencing it: grief, death, debt, falling in love, loyalty to friends and family, etc. Themes are endless. Tap into the collective experience, give your MC a trait or two that your reader can identify with and make your readers care.
Give Them a Conflict or Two
If there is no conflict for your MC, there is no story. There can even be conflict with happy themes like falling in love or having a baby. There can be multiple conflicts. And they do not have to be over the top dramatic.
They Must Evolve
By the end of the book, with the conflict resolved, the MC must grow in some way: either a change in her life situation, a change in her outlook on life or learn something new about herself.
When your characters come alive and jump off the pages, you will grab the attention of your readers as well as agents and publishers.