The great mass of human beings are not acutely selfish. After the age of about thirty they almost abandon the sense of being individuals at all—and live chiefly for others, or are simply smothered under drudgery.
Picking a genre first helps establishes the central question and helps in the selection of conflicts that drive your story. If you have mixed genres it helps with precedence. Selecting a genre also helps with your pitch selection as well as your audience and marketing strategies.
The process of creating characters differs vastly for each author and can even vary for the same author from book to book.