Okay, so the question is: How do you draw the line between deciding to be realistic, and to fudge it in the name of drama, essentially. Pointed out, like, at the end someone gets shot in a way that would normally lay them out but you need the story to keep going. You have a number of options here, and sub-genre and tone is going to have a lot to do with it. The fact that Star Wars is not in any way realistic in its physics does not prevent it from being enjoyed by even a lot of us who understand how bad the physics is, right? Because Star Wars does not present itself as a story where they need to get the physics right. Whereas a hard science fiction movie that we're watching then make an enormous error... I love the Martian, but this is why Andy Weir gets so much flack. It was way better than 99.9 percent of all science fiction stories; he gets one thing wrong and people will notice it because it's a hard science fiction story.
So, you're going to have to decide on the tone of your story, that's a large part of it. And the other thing is, you build up, in some ways, credibility, like I said, often with the small details. And then you use those small details. We often call it the pyramid of abstraction. You lay the foundation with concrete details, building the scene so the reader is on board for what you're doing. And then, when you need to fudge it and strain plausibility, even in a very realistic story, the reader generally gets on board and goes with it, and doesn't let it break immersion for them. In your example, if all the way through this story you had dealt with things very realistically, at the end you even dealt with it realistically, but had the character kind of overcome it for a little while and push through. I am totally on board to buy that, right? I am there with you. I'm like, "This is the climatic moment. Yeah, he should have dropped, but instead, he manages to stand up and push the fire alarm or something like that." This sort of thing, readers will be on board. You just need to make sure to keep them on board and to sell them on the idea of the tone of your story.
As Sidney and Jen pointed out earlier, the wound that is ultimately fatal, often is not immediately fatal. That's one of the other Hollywood tropes that I sometimes hate is: sometimes he gets shot and immediately falls down. No, doesn't happen. A headshot maybe, but that's about it.
Yeah, I was interviewing some people for a story I was writing, and I interviewed someone who had been shot. And he said "It felt like someone had tapped me on the back, just like that, and I didn't know I had been shot." The bullet went all the way through him, but he still was up and doing things for another couple of minutes.