Im writing a short story and have personalities written out for each character but don't know to show them off in the story. Like on character is hot headed and impulsive while the other is more reserved and hides his emotions. Any ideas on how to help?
Create situations or opportunities that require the character's response. The way the character responds should reflect the traits you want to show. In a situation that requires calmness and collectiveness, the hot headed character would just go straight ahead without thinking and regret it, while the shy character is more quiet and hesitant over what to do. But it doesn't have to be a situation. If I were to write a reserved character in a classroom, I'd just describe him sulking in the back all alone. It does the trick.
Look back at some of your favorite books and pay attention to how personality traits are revealed in them. You always want to go to good authors and see how they handled things. This is what's meant by "Learn to read like a writer"—writers learn to write largely by reading, but actively, not passively. Look for how they handle things like point of view, showing vs telling etc. To some extent you can also look at how personality traits are revealed in movies and TV shows, but of course that lacks the dimension of needing to get it across in writing. But still to a large extent it's the same thing. They still have to get across through the characters' behavior what kind of personalities they have. That's done by how they react to situations.
Like @ps102 said, I think the best thing you can do for these characters is to put them in a situation. Start with the what if game is you're struggling for ideas. What if these characters were if a bank that is getting robbed... What if these characters were in a car accident... What if these characters missed the last train... I think you probably get the idea. Characters mean very little to nothing without a story. I'm a short story writer too, and I start all my stories start with a situation. This alone has a way of developing characters. Personally, I try not to separate characters from story. The only important parts of your characters you developed are the ones that will be relevant to the story. I advise against trying to squeeze in aspects of your character that might not fit or be necessary to the story you will write. Readers are only going to need to know these characters as it relates to the story. Also, I suggest being flexible when it comes to writing your characters in the story. I once wrote a story with a character who was a major bitch. But as I was working through revision, I realized this sort of character in this sort of story weren't a match. I reworked the story a little and made the character more sweet and a bit naive. It wasn't what I was originally going for, but it made the story so much better.
