Narrators are people, too. They can be almost invisible, and we foget that they aren't necessarily objective in their thoughts and beliefs. Here are a couple of subtleties to consider:

The unreliable narrator, usually a first-person narrator.
I am reading a novel in which the first-person narrator is cranky, prejudiced, ambitious, and vengeful. No one says that about him, but you can hear it in all his thoughts and actions. So I don't trust his interpretation of events. I'm always wondering what the other characters thought about what happened. This is a deliberate choice a writer makes. In the case of this novel, it shows the effects of the narrator's background, adding depth to the characterization and theme.

The narrator's attitudes
Narrators filter their reports of events through their own lenses. Sometimes the narrator doesn't appear to have any beliefs or attitudes about events or characters; other times they do.

A narrator's biases will be more obvious in a first-person narration, as we saw in my retelling of The Three Bears. But be wary of third-person narrators as well. They select the details they present, and they aren't necessarily objective.

First- vs. third-person
The difference between choosing a first-person and a third-person narrator can be subtle. Consider these possibilities, which I made up:

I ran the last few blocks home, puffing and panting. With every step, I was thinking about ice water. Or maybe iced tea. Better yet, a good stiff bit of something stronger. I was also thinking that exercise is a terrible thing, and I should be content with the body God gave me.

She ran the last few blocks home, puffing and panting. With every step, she was thinking about ice water. Or maybe iced tea. Better yet, a good stiff bit of something stronger. She was also thinking that exercise is a terrible thing, and she should be content with the body God gave her.

What's the difference? How would the difference play out over a whole story? Why might a writer choose 1st or 3rd person?