Think of the millions of people who inhabit the earth. No two are entirely alike. There may be similarities but we're like those snowflakes we learned about in childhood. Every one is different.
As a writer, that leaves you with about a zillion choices when you are developing a character in a story, a poem, a personal essay or memoir. If it's fiction, your creativity comes into play. If you're writing about an actual person, then your memory and observation is what you rely on and perhaps interviews with others who also knew the person.
Let's look at the fictitious character first. There's more than one way to create a character. Some writers make lists for each character, marking physical description, history, emotional side, motivation and even more. The list serves as a guide when putting the character into a story. It helps the writer determine the actions of a character. Why did Melinda neglect to give George the message from Oscar? If there's a list that tells all about Melinda, then the reason is clear to the writer. I'm guessing that it is writers who are 'organized' people who make lists like this. It is also helpful in allowing the writer to get their own mental picture of the character.
Another method is to let the character lead the writer. Introduce the character in the opening pages of a novel or opening paragraphs of a short story and then let the character move on his/her own. Let them guide you to where they want to go. Sound crazy? Trust me--it does happen. Character development in the middle grade novel I wrote several years ago worked exactly that way. My main characters showed me the way.
If your character suddenly does something unexpected as you write, something that doesn't fit into a slot in that list you made, it's OK. You can change a character anytime you like during the writing. They aren't immovable until the book or story is published. If you do change in the middle of the stream, be sure you go back through the earlier pages to check whether this change should be noted earlier, as well. You can't let you character have a severe allergic reaction to shrimp in chapter 14 if you've had her eating at an all you can eat shrimp buffet in chapter 3.
When writing creative nonfiction, you are a little more restricted because you're writing about actual people. If you're working on a memoir, the list exercise can still work. Start with your Aunt Jane's physical traits, then her quirky habits, her emotoinal makeup, her history and possible motivation. Making these lists will get the character firmly in your mind. I think that sometimes memoir writers concentrate more on the events in their lives and less on the characters that drove these events. Give them equal time. Both are important to your story.
I've only touched on the topic in this post. Google character development for writers and you'll find many, more detailed articles.
As a writing exercise to limber your creative muscles today, try writing a paragraph or two with a character having the traits listed below, then add more of your own choosing.Do this with lists of your own making on a frequent basis. Consider sharing with our readers.
1. bald man
2. false teeth
3. intense deep blue eyes
4. rail thin
5. 80 years old
6. retired railroad worker
7. father of 6
8. lives in a nursing home