Many readers (most?) will tell you that your characters are the most important part of your story. Sure, the plot needs a lot of attention and has to be believable and make sense. Yes, setting can be evocative and sometimes almost a character in its own right. Good dialogue is vital. Narration has to be punchy, necessary, and is often used for a change of pace.
But your characters—they drive the whole story. Few stories are told without them. They need to be unusual, compelling, some need to be quirky and difficult. Some writers do whole biographies of their characters, writing down hair and eye color and their favorite food. They list every single thing they can think of about each character.
Isn’t that interesting? When I first saw the clip art above, I thought she was holding a long knife. Must be writing too many mysteries. If you can’t tell, it’s an umbrella. I think . . . She’d make a great character, wouldn’t she?
Since I don’t outline, I also don’t do character bios in advance. When I need a character, I introduce him and sketch in a description. In my notes file, I put down full name, hair and eye color because these are the things I’m most likely to forget later. I am learning to immediately also give the character something to distinguish him from the rest. An unusual name, a different physical characteristic (really tall, or in one case, two different colored eyes, a facial scar—you get the idea), a tic or habit (winks a lot, chews gum, etc.), a certain way of speaking (drawl, in clichés, uses unusual words, and so on), or anything else I can come up with that will work for the character.
After the more superficial things are covered, each major character and some minor ones will need specific motivation(s) to drive them. It’s recommended writers use two motivations that are counter to each other, such as love and jealousy. Another one could be the character is motivated to cover up an affair and run for public office. How about greed and the care of a disabled relative? Just by what you’ve written before the character arrives on-scene, you can usually pick out the major motivation. Next try to pick out another, conflicting one. You do not tell the reader about these motivations, you show her by the character’s action and reactions what the main ambitions are. The character arrives pushing his sister’s wheelchair and by his actions, we can tell he’s devoted to her. Later we see him find a wallet, surreptitiously remove the cash and put the wallet back where he found it and pocket the money. Right away you’ve made the character more than one dimensional, and the reader senses that greed is going to cause him trouble, but the reader is sympathetic to the character because of the sister.
I hope you can see that both tiny details and large motivations are important for your characters to “come alive on the page.” No need to agonize over all of this. I do it all on the fly and somehow it all comes together in the end. The small details are there to help your reader keep track of who’s who. The larger motivations are there to keep your reader interested in the character and anxious to find out what happens next.