That you take what’s available. The trick about “improvised” weapon is that it’s improvised.
Or according to Google’s dictionary:
created and performed spontaneously or without preparation; impromptu.“an improvised short speech”
done or made using whatever is available; makeshift.“we slept on improvised beds”
For example, your character is at a bar and they are about to be attacked or threatened by another angry person (drunk or not). They might grab their beer bottle by the neck and slam it down on that person’s head. They also might grab a mug and hit them with it, they might throw their drink in that person’s eyes before they lunge in to go to town. Assuming the chairs are wieldly and they’re backed into a corner, they might pick it up off the floor and attempt to either hit someone with it or keep them at bay using the legs. It’s going to be awkward, but whatever works.
Think about the environment they’re in, think about what is around in that environment that they have access to, and most importantly what every day household item your character thinks is going to make for a good weapon. The vast majority of these are going to be snap decisions made with limited information while under pressure.
You have the better ones a more experienced character and less law abiding character might choose to carry around with them or keep in their car. Then, it’s more likely they’ll carry whatever they can easily explain away.
For example, would you question why someone would keep a heavy duty metal flashlight in their driver’s side door? Obviously, it’s in case the car gets stuck at night. It’s not because it’s heavy, easy to wield, small enough to hide behind the leg, and great at cracking bones. You can also shine it in someone’s eyes, much in the same way you can use the high beams in your car to blind an enemy. That’s just a pleasant side benefit to a useful tool.
Why does a character keep a tire iron in their trunk? In case they get a flat, right? You’re less likely to think that the character used it to break three people’s arms last week on a collection round.
A car door can be an improvised weapon. Someone approaches to drag a character out of their car and instead of politely waiting, they slam it into them when they get close. Then, when they’re stumbling, crack them in the face or the arm with that heavy flashlight. Or they drag their fingers into the door, then open it and shut repeatedly several times until you crush (or even sever) their fingers. The heavier the car door, the better. In the same way, a door in your own home can become an improvised weapon. The character listens carefully, waiting for them to get close or reach or draw back to break it down, then they open it into them.
If you can pick it up and is decently solid, it can become a weapon. A rolled up newspaper can be a weapon. A plate can be a weapon. A frying pan is an excellent weapon, nice, metal, sturdy, especially one that’s been sitting on a hot range or if one were cooking anything in it.
A backpack carrying text books or any heavy books can become a weapon.
Car keys are a weapon.
A broom is a weapon.
Your character can be as safe or as vicious as they want. Play a game with yourself, look around the room and ask “if I were being attacked right now, what would I grab?”. Then, ask your character that question, think about it from their reaction and their background. Think about what they’d do or what they’d choose if they started with nothing and it had to be right now.
Think about where they are and what’s around them. Think about what they have access to. Think about their environment. Then, don’t ask yourself “what would be the best choice for this situation” because your character doesn’t have the time for a “best choice”. They’ve got time for “I see it, I grab it, I hope it helps”. (Also, “I hope I know what to do with it once I have it in my hands”.)
The problem that comes in with a lot of writing of violence is that you can’t munchkin it. Well, you can, but it kills all tension. The advice Toph gives Aang in the second season of Avatar: the Last Airbender when teaching him earthbending. There is no cool move or trickety trick that‘s going to get you through that rock, you’ve just got to dig in. Commit. Write the scene. They’re in it now, they’ve got to be the ones to get themselves out.
One of the hardest parts of writing in general, not just violence, is that sometimes the story doesn’t go the way you expected or planned. Sometimes, it takes off in a whole new direction. Instead of fighting it, let the current take you. See where you end up. Run with it. Live in the moment.
So what if your character grabbed a useless weapon or what they’re trying to improvise doesn’t work out? It doesn’t necessarily mean they’re going to die. (They could.) It’s more a question of what they do next rather than trying to brute force the situation into one that works to their benefit.
A character is defined by what they do when things go wrong, when things don’t work quite right, how they adapt to their changing environment. Their resourcefulness, their cleverness, their ability to maneuver or even fuck up a situation. Their actions could make the situation better, but they could also make it worse. They can make mistakes. They can screw up.
That doesn’t make them bad characters. Or say anything bad about them if they aren’t particularly good at fighting. What makes a badass a badass is how they deal with situations, not the fact that they can kick the ass of everyone in the room. Your character will always have to earn the title anyway, rather than starting with it as a default. So long as the mistakes your character makes are incorporated into their development then it doesn’t really matter.
Take chances. Make mistakes. Put them on the defensive. Make them think. Make yourself think. Imagine different ways out of a bad situation. Think about the consequences. If your character does X how do the other characters respond? Do they get angrier? Do they become scared? How do they try to take back control? Which one feels like the right course for this character? Imagine it from the perspective of a different character. How would they do it differently?
Think about it. That’s pretty much all you need to do.