This tip can be summed up in a simple question. If you don’t like your characters, why would anyone else like them?
Did I say a simple question? Actually, this advice isn’t simplistic at all. Lots of writers think they’d never create a character they didn’t like, then they go right ahead and do so because they don’t realize that this advice applies to all of their characters, not just the main ones.
Even villains are part of the story. Even minor characters flesh out the plot. So you’d better like each and every character you write about, even your villain, if you want your readers to care enough about the story to become a part of it. After all, a story is a world that readers enter for a brief period and why would they voluntarily go to a place filled with characters they’ll dislike?
Make some aspect of the most dastardly villain likable. It’s easier than it sounds because no one is ever one hundred per cent bad. Make them bad but make them realistic.You don’t need to go into detail about whatever turned them to the dark side. Just write a sentence or two to sketch a side of them that’s decent. Maybe they love cats? Small children? Sunsets?
Then look at your throw-away characters and do the same thing. Make them real, make them flesh and blood, even if their part takes up less than a page. It only requires a sentence to make a minor character memorable, sometimes only a phrase within a sentence.
All of your characters will then be be interesting enough for your readers to finish your book in one setting and ask when your next one will be published.