In other situations, though, it comes out very strangely: "I'm startled when the guitar explodes." That makes it sound like I'm startled now. But if I'm reviewing or talking about a book, and I describe the book's events in the present tense, it doesn't seem right to suddenly shift to past tense for a sentence to avoid this: "They rush back into the castle. Then the guitar exploded, which startled me." It's maybe even worse to shift within a single sentence: "They rush back into the castle. Then the guitar explodes, which startled me."
I suppose a simple solution would be to put everything into past tense - "They rushed back into the castle. Then the guitar exploded, which startled me" - but I remember learning at some point that this was Not the Done Thing for describing events in books/movies/etc.
How do you handle this? Am I weird for wondering about it? I guess I write a lot of book reviews . . .