Theatrical superstitions are superstitions particular to actors or the theater. Acting being a chancy business, there are a lot of these.
Shakespeare's play Macbeth is said to be cursed, so actors avoid saying its name (the euphemism "The Scottish Play" is used instead). Dire consequences are said to befall anyone who doesn't use the euphemism. It is also said that Macbeth has never been staged without at least one of the actors being killed or seriously injured during the run. There is no hard evidence for or against this, but it is worth pointing out that the play includes more fight scenes and other such opportunities for accidents than the average. The origin of this superstition is unknown, but its continued popularity is probably due to the juicy stories of doom that grizzled actors tell newbies.
It's unlucky to wish good luck upon an actor or a production. Instead, one should invite disaster; the phrase "Break a leg!" is most commonly used. Again, the origin of this is unknown; it may be a relict of a more general superstition against wishing someone good luck.
For similar reasons, actors should not be given flowers before a performance. After is ok.
One should always leave a light on in an empty theater; this wards off ghosts (hence the light is often referred to as a ghost light). This superstition has practical value. The backstage area of a theater tends to be full of clutter, so someone who enters to find the theater completely dark is liable to injure themselves while hunting for the light switches. (This would have been a worse problem before the invention of electric light.) Theaters that have stood for more than a few decades tend to have lots of associated ghost stories; more than other public buildings of similar age.
Another ghost-related superstition is that the theater should always be closed one night a week to give the ghosts a chance to perform their own plays. Traditionally this is Monday night. This conveniently gives the actors a day off right after the weekend performances.
Never whistle or clap on stage. This is likely to derive from history; in the past the stage crew would use coded whistles or claps to cue scene changes. Actors who whistled could confuse them into doing something at the wrong time, which could in turn get someone clobbered with a piece of scenery. Nowadays, the stage crew normally uses an intercom system.
No real money should be used on stage. This might derive from gamblers' superstitions about money, or it could just be a sensible precaution against theft.
No live flowers or mirrors on stage. This is pure practicality; flowers will wilt under stage lights, and mirrors will reflect stage lights at the audience if the set is not designed with extreme care. (The musicalA Chorus Line has a famous mirror scene; it can be done.)
It is bad luck to complete a performance of a play without an audience in attendance, so one should never say the last line of a play during rehearsals. To get around this, some production companies allow a limited number of people (usually friends, family, and reviewers) to attend the dress rehearsals.
A bad dress rehearsal foretells a good opening night. This is probably sour grapes.
No wearing of green unless absolutely necessary. This may have been because green looked awful under old stage lights, or because green clothes may have been associated with prostitution in Elizabethan England (see Greensleeves).
