Character studies are reason number eight and it ties back to socialization from earlier in the month.
Listening to people as they relay their daily lives or memories can help you shape realistic characters.
What seems like a romantic gesture to one woman, another woman may find it very inadequate. That could be inspiration to write a scene in which your hero contemplates doing something for the heroine but remembers a bad reaction from an old girl friend.
No two people react the same to any situation. Watch what happens to a change in policy in the work place. Some people openly and angerily voice their opinions on why the change isn't fair. The next person may say nothing but you can tell by their sour expression that they don't agree and the next person may just shrug it off. These reactions and the body language that goes with them will help you flesh out a character and how they react to any time of change in their lives.
Getting flawed but sympathic characters is every writer's goal but when we only have our feelings to base a situation our characters can become flat. Imagine reading a book where every character reacted the same way to every situation. There'd be no room for growth on the character's part. No one to set an example or give a different perspective to the main characters. Working a day job gives you access to all personality types and insights on how to create a diverse character.