Each fictional character should have a full spectrum of personality traits based on his or her background and experiences. A character inventory sheet is a good place to start but I must admit I don’t always pull these people out of thin air. I use a number of brain starters - places you can look for descriptions of character traits, and some ideas about what traits generally travel together. None of these is THE answer, just a place to start.
Yourself: Yes, every character will have some of you in him. It’s okay for one or two to be mostly you, but you have to stretch yourself. Add one trait you’d like to have or magnify a trait you dislike about yourself and you’ll be amazed at how different that person becomes. A twisted funhouse mirror image of you is a fun character to play with.
Your family members: These are the real life people you know best. You’ve seen them at their best and at their worst. If you choose a brother or cousin don’t sugar coat them. If you’ve been close to an alcoholic, a drug user, an adulterer or a bigot and still love them anyway then you can show them to your readers in a way that will help the care about that flawed character. Just be sure to fictionalize them enough so that they are not recognizable. You can put that familiar personality into a whole new body, and that’s usually enough.
Public contact: The people you see in bars and restaurants, on the bus or in line at the Department of Motor Vehicles all have stories that we can only guess at. But they each show us a piece of their personality and if we are observant we will see traits that we can add to fictional characters to make them more familiar and believable.
Other People’s Books: I may hear disagreement from my peers here, but I say don’t be shy about borrowing personality traits from your favorite fictional characters. If another writer developed a person with a consistent and interesting personality you can use the same traits but drop your character into a different situation. In some cases you might even use someone else’s character whole. Excellent novels have been written featuring Dr. Jeckyll’s maid (Mary Reilly by Valerie Martin) Captain Ahab’s wife (Ahab's Wife or, The Star-Gazer by Sena Jeter Naslund) and Sherlock Holmes’ favorite woman (The Irene Adler series by Carole Nelson Douglas.)
Newspapers and magazines: You can use any interview you find useful as the basis for a fictional character. One plus is that some of the usual research becomes unnecessary. Often people written up in The Washingtonian become thinly disguised minor characters in my work. Magazines like Men’s Journal and Redbook often carry short character sketches as part of a feature promoting clothes or cool new toys. Why not use them.
Myers-Briggs Type Indicator – This personality test divides humanity into 16 types using four parameters. According to this test we are all either extroverts or introverts, intuitive or sensing, thinking or feeling, and we either make judgments or open our perceptions. I’m not sure if I believe the science, but it doesn’t matter. Each of the 16 types is a collection of personality traits that’s easy to work with. You can create a character by choosing any one of them.
Horoscopes: Personally, I think this is nonsense too, but the typical description of the Ares male does sound an awful lot like me. These are familiar trait sets, and it’s a place to start, which is the point.