I find that figuring out how a character reacts to pain is a great way to flesh out a character. It's usually just a thought experiment, though occasionally I'll write a brief scene to help me solidify the things I learn about that character. For example, I'll put the character in a situation that would cause the "average person" some sort of emotional pain - the death of a family member. Finding out how the character reacts to that brings up a ton of questions right away:
- How close is the character to the family member in question?
- How does the character react externally, in public?
- How does the character react privately?
- Is there anyone that the character would feel comfortable expressing their pain to?
- What's the first thought that runs through the character's head when hearing the news?
There are stories that can grow out of this information right away - if the character isn't close to the family member who died, why? Was there some sort of estrangement? If the character breaks down in public, how do the people around the character react?
Physical pain is less similar that it would seem on first blush. Speaking from personal experience with chronic pain, I know that I can "push through" pain I'm used to, but will be laid out flat by something unexpected. I have chronic head pain, so I'm used to working around headaches; however, if I throw my back out, the slightest shift in movement will leave me incapacitated. So ask yourself:
- What kinds of pain your character can work through, and what will drop them into the fetal position?
- Does your character react differently if someone witnesses an injury, as opposed to being alone?
- At what point will your character ask for help?
In general, people don't like to think about pain. However, seeing how someone reacts to different kinds of pain can help give a character depth, even if it never comes up explicitly in the text. Try not to torture them too much, though - you don't want them to run off screaming into the night.