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For an example of the first, suppose aliens kidnap their fantasy characters (Star-Lord style) and insert them into an SF genre. Players who really weren't in the mood for (or always dislike) Big Space Ships would be disappointed. They don't get to play the game they thought they signed up for.

So, for your particular example, if you want to kill off what the genre/style of the game would seem to indicate is a fixed character, you have to avoid those two things:

For an example of the first, suppose aliens kidnap their fantasy characters (Star-Lord style) and insert them into an SF genre. Players who really weren't in the mood for Big Space Ships (or who never like them) would be disappointed. They don't get to play the game they thought they signed up for.

So, for your particular example, if you want to kill off a character that the genre/style of the game would seem to indicate is fixed, you have to avoid those two things:

Finally, if the players know all along that their expectations are likely to be subverted in some unknown way (because you've told them that's what you like to do or shown them that in your previous games) then they'll find it easier to follow the change when it happens. Knowingly playing a bunch of rubes whose minds are about to be blown is often more fun than being a rube whose mind is about to be blown, but it's a different (and perhaps lesser) form of expectation-subversion when it actually happens.

A surprise which makes the players think, "ah, now I understand!" is usually going to be easier for them to get behind than a surprise that makes the players think, "what? None of this makes sense any more", although the latter is more challenging. So if your big reveal can release a tension ("OK, so that's why the mooks have all seemed kind of unhinged: they're cultists!") as well as create one ("uh oh, we are really out of our depth with these extra-planar horrors") then it will feel more logical and probably more fun.

Finally, if the players know all along that their expectations are likely to be subverted in some unknown way (because you've told them that's what you like to do or shown them that in your previous games) then they'll find it easier to follow the change when it happens. Knowingly playing a bunch of rubes whose minds are about to be blown is often more fun than being a rube whose mind is about to be blown, but it's a different (and perhaps lesser) form of expectation-subversion when it actually happens.

• Make sure the players aren't counting on that character for their enjoyment of the game. This usually just means making sure there are plenty of other sources of fun, and fun to be had in reacting to the death. But a player who thinks of the game as being about their character's relationship with X might well see the death of X as the beginning of the end of the campaign, and basically figure their plot arc is complete once the consequences of X's death are played out. This more or less rules out killing a PC solely for the purpose of genre-subversion -- I'm not saying it can't be done, but it's liable to disappoint at least one player and you'd need to compensate for that by ensuring it's more than just a "gotcha!" moment.

• Make sure the death doesn't cause the players to question whether they're playing the game right. If they care about X, they might suddenly go max-defensive on every NPC they care about on account of the game having "punished" them for not defending X, and they might feel the "punishment" was unfair if nothing in the game indicated that X was vulnerable. Unless that's really the way you want the game played, you have to be careful not to provoke the players to play it that way.

• Make sure the players aren't counting on that character for their enjoyment of the game. This usually just means making sure there are plenty of other sources of fun, and fun to be had in reacting to the death. But a player who thinks of the game as being about their character's relationship with X might well see the death of X as the beginning of the end of the campaign, and basically figure their plot arc is complete once the consequences of X's death are played out. This more or less rules out killing a PC solely for the purpose of genre-subversion -- I'm not saying it can't be done, but it's liable to disappoint at least one player unless you compensate for that by ensuring it's more than just a "gotcha!" moment.

• Make sure the death doesn't cause the players to question whether they're playing the game right. If they care about X, they might suddenly go max-defensive on every NPC they care about on account of the game having "punished" them for not defending X, and they might feel the "punishment" was unfair if nothing in the game indicated that X was vulnerable. Unless that's really the way you want the game played, you have to be careful not to provoke the players to play it that way.