In The City of Ember, we only get a glimpse inside two character's brains for the most part. These two are, you guessed it, Lina and Doon. Through them, we get a glimpse of what life is like in Ember, and we're also treated to their thoughts as they solve the mystery of Ember's existence.
One of the nice things about this narrative technique is that we get a fuller picture of each character. Lina, for instance, thinks of Doon when she is getting closer to figuring out what the instructions from the Builders contain: "The message had something to do with the river, a door, and the Pipeworks. And whom did she know who knew about the Pipeworks? Doon, of course. She pictured his thin, serious face, and his eyes looking out searchingly from beneath his dark eyebrows… he was curious. He paid attention to things" (7.97-98).
Doon, of course, wouldn't be sitting there thinking about his own thin face and dark eyebrows, unless he's got a secret vanity problem. But since Lina does it, it helps us to get a good picture of how Doon looks, and we also know which of his behaviors (like curiosity) get picked up on by other people. Doon also takes pride in being curious and a fast learner, so the fact that Lina knows this about him too shows that his behaviors are right in line with his values.
This isn't true for every character in the book, obviously. There's the mayor, who lies to the people of Ember while hoarding goods for himself. We never see inside his head, so we don't know how he explains that to himself—and honestly, we kinda don't want to be in his head, since it must be a lonely, nasty place.
The overall effect of this narrative technique is that we get a good understanding of Lina and Doon's motivations, but we only understand the other characters as much as they open up to Lina and Doon. People like Clary, Poppy, and Lizzie are practically open books, while others like Mayor Cole and Looper remain mysteries.