Basically, common sense. Makes sense (pun unintended), but it shouldn't be gamey, because we have the game for that. I prefer a good story, that's what we're here for. It'd have to be case by case, in my opinion. But most of the time, yeah, if you're under a falling, burning car, you're roadkill. Still, depending on alot of factors, you'll be surprised what a person could get out of. For example, an injured alien is guarding a place, and you decided to come out with a few guys, chances are, the alien might not do anything. Stuff like that. Basically, all I'm trying to say is, don't go with game logic, go the direction writers go.
Oh, and another thing. I'm a big horror fan. :) It would be great if this XCOM roleplay is a horror. I think XCOM is meant to be a horror, the world's first horror strategy game, haha.
Anyway, I'm still unsure if I'd join. I've always been here to advise, as I can empathise with you wanting to start an XCOM roleplay. I'll play by ear.