What must occur between the subject and observer for the subject to see exactly what the observer sees in the mirror?

Answer

Both parties would need to share an identical line of sight relative to the mirror surface

For the subject to see the same image as the observer, both parties would require an identical line of sight relative to the mirror plane, a condition usually prevented by the geometry of these arrangements.

What must occur between the subject and observer for the subject to see exactly what the observer sees in the mirror?
Venüsplaneteffect