Plot Ideas, Writing Prompts, Character Building: What can I write?

An awful day: Character building

Everyone has days where there is this one person who has gotten on your nerves and continues to bother you all day. What do you do? You turn them into an antagonist, of course.

Let's say there's this guy and we'll call him... let's call him Bob. Bob works at a firm and is... how about he works a desk job, and while he's on his break, this other guy, Joe, comes up and with a laugh, a joke, and a slap on the back, he takes the cup of coffee Bob was making for himself. I don't know about you but when it comes to food or drink, I don't particularly like to share. Well, Bob is like-minded, and coffee is on the list of things neither of us likes to share. Later, at lunch, Joe takes the spot in the building's cafeteria that Bob was aiming for. Then he hears Joe laughing and flirting with a coworker Bob has a crush on, and she doesn't seem to mind. Well, if that were you, I doubt you'd be very happy with Joe. So Bob goes home and calls his sister and complains about his day, and about Joe, turning him into a supervillain who stole the crown jewels, usurped a place in parliament, and kidnapped the heroine.
Of course this kind of exaggeration isn't something you would really hear from adults, but you get the idea. Even better? You can get your protagonist the same way too.
Bob has a niece he adores, and she loves horses. His nephew has a huge collection of books about the military. Bob has a love of all things historical.
I know, these things don't seem to have anything to do with one another, but hang in with me here.
During an online creative writing course, Bob is tasked with creating a character. What does he do? He comes up with a protagonist who is fighting in WWII and finds an injured horse in the ravine he's hiding in after he got separated from his platoon.
By taking elements from existing people's personalities and likes/dislikes, changing them up a bit, and making sure that "any similarities to person's living or dead is simply a coincidence as all characters are from the author's imagination" is not a lie, you have your very own protagonist!
Of course, to make sure that you don't steal someone's life and make a book out of it, you must only inspire yourself from these people around you. You really shouldn't just go and read over someone's diary, copy and publish it, unless you really want to find out what it's like to be sued.
As for more in-depth personality for your character, use your own life. If you had a bad experience with let's say, small spaces, give your character claustrophobia and use your own experience to create a backstory for that fear. Now you have a fear that could lead to many kinds of interesting situations, like the only escape from a trap being a small tunnel in the basement, or sometime when the character must hide in a dark cave. This gives you a chance to deepen your character, show their weakness or strength by being able or unable to conquer their fear.
If you think back to books you've read, you can usually find at least one case like this, if not many. It is a very useful tool for character building, and also gives you opportunities to add details to your plot.
That's all for now, but next time you have to come up with a character, use this technique and see how it works for you!