Many books start with a character. What anchors the writer's attention may be a few words of overheard conversation, the way a friend keeps smiling as she screws her life up, the look in the eyes of a face in a photograph. Occasionally an entire person will simply walk out of your imagination, so fully realised that you know them as well as your oldest friend and never hesitate in deciding what they will do or say. More commonly, only one quality of the protagonist's character will be clear at the outset. The creature will be more graceful if you allow it to evolve from your imagination.
Some writers go in for outright bodysnatching, putting a whole known person in their story.
This is no problem with minor characters, but you may find that you only want the truth that fits, that you need your central character to do things which the real model would never do. Abstract or philosophical thinkers may begin with the theme of the book and develop a character to act it out, and those whose technique is very objective may start with an event in the plot. Working this way, from structure to character, can still create a rounded, credible person. By the time you are happy with your scenario and have written a synopsis, you should also know your protagonist well enough to be able to write the key elements of their character down on a card, add a picture to it and pin the card up over your desk to be used to keep them acting in character as the story grows. If the protagonist keeps falling out of character, you may have grown attached to a personality which is incapable of acting in such a way as to make your story happen.