Best to not look in this looking glass....
Walter knew it lived in the mirror and so for three nights he cast glances into it. He didn't know why it was only there at night, but it was. Furthermore he had to surprise it or it would simply disappear. He thought of calling Dr Rosalie, but she wouldn't believe him. A few weeks before he had asked her to increase his medication but she refused. He knew she would think he was making up a story to get his way.
Finally he screamed at the mirror: 'I know you're in there!", and that's when it appeared.
At first the conversations were little more than small talk, but then It began telling him things. They were things about the people he knew. They were bad things about people he knew and which made Walter angry. It began telling Walter what he could do to 'fix their wagons'.
That's the phrasing it used when it was helping Walter. He explained as much to Doctor Henninger, the psychologist assigned to Walter after the untimely deaths of Dr Rosalie, the tall boy who would skateboard at the end of the street, and the neighbor who called Walter that name he'd always hated.