Although general rules of theatre apply, a trip to the opera has its own pitfalls and particulars.
Golden Rules for Opera
Be sure to applaud only when the conductor takes to the podium (at the very beginning and after the interval), after the overture (the musical introduction before curtain-up), at the end of an act, after an impressive aria and at the final bows. Avoid whooping, whistling or braying - feet stamping is a definite no-no.
Surtitles provide an electronic rolling text of a summary translation above the stage and can help you to keep up during the performance.
Top Tip: familiarise yourself with the story (often overly-complicated and slightly ridiculous), especially if you are in the company of regulars.
Country House Opera
Performances begin in the late afternoon or early evening, but usually a little earlier than conventional theatre. There is an extended interval (approx 90 mins) during which the picnic is enjoyed.
Top tip: take picnic chairs to sit on as it's especially difficult to sit on a rug elegantly when wearing an evening dress. Also, take a warm shawl to fend off the chill of the British summer evening.