How would a character with PTSD act?
That depends on numerous factors. Age, type of trauma, severity and the length of time. A little girl in a car accident with injuries will present differently from a combat veteran or survivor of a tornado outbreak. However, there will always be a core of symptoms.
Celeste Carlson is 15. When 13, she was in an accident that broke her arm, ribs and punctured a lung. She was in for an unpleasant hospital stay and almost had to repeat 7th grade. Compounding the accident, the girl driving was killed at the scene. Celeste refuses to ride in small cars, will only ride in SUVs and straps in like it's the space shuttle. She cries easily, is triggered by traffic, and often has nightmares. When Celeste gets rattled, it can take a good while for her to get back on balance(minutes to hours). She may often be seen as distracted.
Nick Morrison is 44. When he was young, he decided it would be a good idea to join the Marines. Then Iraq, part 2 happened. Nick was part of the Mean Green Machines' Shock & Awe campaign. Iraq part 2 was a different war than the original Desert Storm. They tried to do it on the cheap with a Coalition of the Willing so they saw very heavy fighting. He was probably going to do OK until Fallugah and an Improvised Explosive Device that put him out of action for six weeks. Twenty years later, Nick works as a field engineer and travels a great deal. He's married, but it's rocky. He's jumpy, loud noises can rattle him, and he has nightmares. His wife suspects he is cheating on the road, but the truth is he's faithful and uses the alone time to drink.
Scooter McGee is 25. When he was 17, an EF-3 tornado smashed into their rural Arkansas community at three in the morning, killed 22 people and blowing him into a tree on a mattress and breaking his arm. Scooter started drinking before the cast came off. After graduation, he became an apprentice electrician. He's a functional alcoholic. Bad weather gives Scooter the shakes. His old friends wonder what's up with him because all he does is work and see his girl, and go hunting and fishing to cover up his weed and alcohol hobby.
The hypervilligence part of PTSD is exhausting. It's like you are constantly on red alert. This is why people with PTSD are often described as twitchy. They may be too tired to sleep (insomnia).