In the introduction to a 1997 edition of I Am the Cheese, Cormier admits that the book "began as [...] an experiment with first person – present tense [...] simply because I had nothing else to create." He intended for the book to be completely in the first person – and he based the boy in the story off his own childhood self – but later he was inspired to intermingle a past tense narrative. After he finished, he became worried: "Who would read this complex, ambiguous story?" The answer: lots of us.
Two, almost three, narrative techniques are used in this book. The first is first-person (central narrator), where the narrator – Adam – is telling his own story from his own perspective. This point of view gives us a really intimate look into Adam's thoughts and helps us sympathize more with the events that happen to him. Cormier does a wonderful job making Adam a relatable, if slightly neurotic, character whose fears and worries we understand.
The second narrative technique is third-person (limited omniscient). This means that it's no longer Adam telling the story, but the narrator has a really good knack for knowing what Adam (and only Adam) is thinking and feeling. While we're not hearing these experiences in Adam's own words, we are still seeing them mostly from his point of view. You'll notice, for example, that we never get a chance to peek inside the mind of Brint.
Littered throughout this narrative technique there is also transcribed dialogue; the author has typed out exactly the words that are being spoken by Adam and Brint. This is almost like a play on a stage, except that instead of the author telling the characters what to say, the characters are providing the material and the author is just writing it down.