You've read Prince Caspian, right? If you haven't, go read it. (Or at least watch the movie.) Then come back and finish.
There's a scene where Lucy sees Aslan, the giant talking lion, and he beckons for her to follow him on a course that doesn't seem to make sense. No one else in the group sees him, and they basically don't trust Lucy enough to do what she says. She's the youngest, etc. etc.
So they go the other way, run into all sorts of trouble, waste all kinds of time, and Lucy sees Aslan again—and no one else does. Even though he's right there! Eventually, they come around and everyone sees him.
You, the reader, never doubt that Lucy actually saw Aslan. You know they should follow him as surely as she does. Are you simply more trusting of young girls who see things you don't? No. You know she saw Aslan, because you saw him too.
A few reasons for this:
The book's in third person, and the narrator never leads you astray. You trust the narrator when he says Lucy saw the lion.
You trust Lucy. You know her character, and that she wouldn't ever make something up or follow a whim. She's not prone to hallucinations, either, even though she's frequently accused of being so.
And finally, Lucy knows what she saw and doesn't doubt it.
So when Lucy sees Aslan, you trust that she's not imagining or making it up, and you trust the narrator to tell it like it is.
In The Freezer, I have a narrator and character who are one and the same. When extraordinary things happen to him, I want the reader to be on board. They have to experience it with him. This example helped me realize a few flaws in the way I'd written a couple of important scenes.
Mostly, I realized that if my hero doubts his own senses, the reader probably will too. This can't be a case of unreliable narrator at all, and that has implications for everything from how he tells his own mistakes to what he's thinking at any moment.
And that leads to choices about where the tension in the scene will actually be—is this really happening, versus what he's going to do about it—but that's another blog post.