Mirrors in our homes bask in muted and limited lighting sources that do not cause much glare. On the stage, however, lighting is very intense and arrives on stage from as many as a hundred different directional sources. Mirrors on stage, therefore, reflect very intense light back into the eyes of performers and audience members. They steal focus. In addition, mirrors can destroy important stage illusions. Directors often plan staging for the benefit of an audience placed in front of the stage, and the audience's view is restricted to a "front only" perspective for a reason. When a mirror is included on the stage, the audience suddenly is able to see around and behind the actor in a manner that may not be best for the performance. For these reasons, set decorators, lighting designers and technical directors must take steps to reduce the impact of a real mirror on stage.
One way to reduce the negative effect of a mirror is literally to blunt its reflective power. Mask a real mirror with an application of a dulling agent such as hair spray. There is also a product that simulates "window frosting" available in handy spray-can form at most home-improvement stores. Another option is to create masking from a single large sheet of white frosted gel media cut to match the size and shape of the mirror.
If the play script refers to a mirror either in stage directions or in dialog, you must provide a mirror. The mirror, however, need not be visible to the audience. Experiment with furniture layouts. Point a mirror toward the wings or a side wall of the set rather than back toward the audience. Sit in various seats in the theater and check the sight lines with all lighting instruments turned on. Adjust the mirror until the majority of the lights no longer reflect back toward the audience.
Of course, the most effective treatment is no mirror at all. If the set design is suggestive rather than realistic, remove the mirror from its frame and let the audience imagine the mirror. This trick works only if the mirror frame makes an obvious suggestion, such as a large frame attached to a vanity or a mirror frame attached to the top of a dresser. Hanging an empty frame on a wall is not specific enough: Audience members will wonder why the actor is talking to an empty picture frame. Another strategy is to replace a mirror with a sheet of silver-colored poster board. Silver paper mounted inside a frame will read as a mirror.