The brilliant Lillian Groag has been a favorite stage director for the Virginia Opera for the past 20 years. When former VO Artistic Director Peter Mark convinced her that her pristine reputation as a theater director, playwright and actor was an ideal requisite for tackling opera, she plunged into “Tosca” and loved the experience so much that she has returned every season since. Her wit and imagination are the forces behind the company’s latest production of Mozart’s “The Marriage of Figaro,” coming this week to George Mason University’s Center for the Arts.