Self-insertion is a literary device in which the author writes themselves into the story under the guise of, or from the perspective of, a fictional character.
[1] The character, overtly or otherwise, behaves like, has the personality of, and may even be described as physically resembling the author of the work.
Among professional writers, the intentional, deliberate use of first-person and third-person self-insertion techniques are commonly considered to be an unoriginal action on the author's part, and represents a paucity of creative thought in their writing.
[4] Self-insertion can also be employed in a second-person narrative, utilizing the imagination of the reader and his suspension of disbelief.
The reader, referred to in the second person, is depicted as interacting with another character, with the intent to encourage the reader's immersion and psychological projection of himself into the story, imaging that he, himself, is performing the written story.