I don't think that we have a set of individual characteristics that make someone more or less prone to developing PTSD.
Very quickly, consider the following two possibilities. An officer approaches a scene after a week where they've been well rested well supported by their team, when no one has been off sick, and they've had lots of resources available to them. If they come upon a car accident, they're going to have a very different interaction with that traumatic exposure than they would if they've had a week that was really hard, in which they hadn't slept much, and they'd been having trouble at home and had been working too much because somebody had been off sick. Now instead of the car looking unfamiliar, the car is the same colour as their spouse's car; the kids looks very much like their kids, and everything becomes familiar, but it's not them.
You have the same human encountering two different traumatic exposures and they're going to have two very different responses, as I'm assuming you can intuit. This situation makes identifying specific individual variables very difficult to do. We can give you broad strokes, and we hope to be able to do that fairly soon, in the next few years, but saying this person is always more vulnerable or this person is always less vulnerable is not something I think we're ever going to do.