The narrator interrupts the story to comment, judge, offer an opinion, or speak directly to the reader.
In One, No One, and One Hundred Thousand
I wasn't myself, I can assure you. So much so that a moment later, before going out, I barely turned my head to look at him again in that mirror, and he was already another, also to me, with a diabolical smile in his sharp, shining eyes.
"It wasn't me, I can assure you" interrupts the account to speak to the readers and answer an objection they have not yet raised. The narrator gets ahead of the reader's doubt and argues with him, as in a conversation.