Quirks, preferences, odd little habits - they're what make real-life people distinctive and interesting, and they're the secret to making fictional characters vivid and convincing. What do your main characters like, love, detest? What do they get up to when they're home alone? What do they eat for breakfast? Describe your key characters early on - So your reader gets a sense of their age, occupation, looks and interests - and make darn sure they're interesting. Lacking inspiration? Observe the people who fascinate you, and, if you like, pinch a few of their tics, penchants and most-used phrases. But don't just write them into your novel unchanged, or they might read it and sue you, or stop inviting you to dinner. Ask Ernest Hemingway.