One thing I’ve run into in a lot of comments is “but why doesn’t character X see what’s obviously going on?”
Simple. Because every character is a person – and people don’t have perfect knowledge of other people’s actions and intentions. Not ever.
Unless you’re doing third-person omniscient POV, there’s no such thing as a completely objective narrator. If you’re writing someone’s point of view, third person or first, you are writing what they see and believe. And any one character is not going to see or know everything that’s going on, even if they’re impartial as a judge. On top of that, if they’re a main (or even good supporting) character, they are not impartial. They can’t be. The events they’re caught up in are life-or-death important. They’re not going to stand around going, “Gee, what did Y really mean by that, and did I misread his intentions?” Nope. They’re going to jump in with their best guess and start swinging.
And sometimes, their best guess is wrong.
Readers have it a lot easier than characters. They can see inside everyone’s head, compare and contrast, bring in outside knowledge, and – most important of all – put the book down and think.
Something our poor beloved characters really don’t have time for as they’re trying to fight off the slavering jaws of alien death. Or even trying to sort out a whole bunch of refugees from another world.
Unreliable narrators are tricky enough in a regular story, but in a crossover, they become both more important and even trickier. Because the characters from each side are missing a lot – a lot – of information on their opposing numbers. And based on the background any particular character is coming from, they may not know how much they don’t know. Throw in any amount of tension or threat, and people are going to fall back on mental shortcuts of “this is how I deal with X”, just to stay alive. Because your brain can only keep track of so many things – and people! – at once.
FYI, there’s an actual reason the Five Man Band trope is such an enduring staple of fiction. Studies of people in high-stress situations confirm that four other people is about as many as you can keep track of when adrenaline is pouring through your bloodstream. Any more than that, and things get sticky. That’s not opinion, or even training. That’s a fairly hardwired biological fact.
So one of the things I check and recheck my writing for is, who knows what? Or, who’s likely to know what? And then, what would they do with that? Does Guy A know the details of the life or death situation Character B just escaped by the skin of his teeth? If he doesn’t, does he have some reason to find out? On top of that, does Character B have any reason to realize A doesn’t know what B knows by heart?
If not… well. Communication is hard.