I feel as though sometimes, particularly when fighting some of the bosses in the Original Mode, that the words I need to type to kill zombies are becoming more difficult in response to my performance throughout the stage, even though I always play on the same difficulty(Normal). Am I mistaken, or does the difficulty change depending on the stage path?
You're absolutely right! It's wholly dynamic. If you never took damage or had to use continues, the difficulty would climb and climb until you always had to type a long sentence in. The way I remember, it lowers a little with single lives you've lost (i.e. taking a hit) and drops considerably after continues.
The tranquilizer is also supposed to have such an effect. No reason to believe it doesn't.
Oh and the path you chose doesn't directly affect the difficulty, but you can be led to slightly different boss fights, i.e. several bosses have types A and B one of which could be tougher to beat than the other. But that's unrelated to the first point.
I used to play the game a lot so I'm fairly sure of this.
Turns out it's because of the tranquilizer power up I pick up occasionally, which, according to the game, weakens the zombies temporarily.