Theatrical superstitions

Theatrical superstitions are superstitions particular to actors or the theatre.
Macbeth.
William Shakespeare's play "Macbeth" is said to be cursed, so actors avoid saying its name when in the theatre (the euphemism "The Scottish Play" is used instead). Actors also avoid even quoting the lines from "Macbeth" before performances, particularly the Witches' incantations. Outside a theatre and after a performance, the play can be spoken of openly. Saying the names of the characters, including Macbeth, seems to be fine. If an actor speaks the word "Macbeth" (when referring to the Scottish tragedy, not the character) in a theatre prior to one of the performances, they are required to perform a ritual to remove the curse. The ritual may vary according to local custom: one is to leave the theatre building or at least the room occupied when the name was mentioned, spin around three times, spit, curse, and then knock to be allowed back in. This particular iteration of the ritual is documented in the play "The Dresser" and its film adaptations. Another example is in the song 'Take a Break' from the hit broadway musical Hamilton. Alexander refers to 'the Scottish tragedy' in a letter to Angelica Schuyler, comparing himself and his enemies to the characters in the Shakesperian tragedy. 
One version of this legend claims that it was the actor who played Lady Macbeth who died during the play's first production run and that Shakespeare himself had to assume the role. There is no evidence that this legend is factual.
"Break a leg" vs. "Good luck".
Generally, it is considered bad luck to wish someone "good luck" in a theatre. Prior to performances, it is traditional for the cast to gather together to avert the bad luck by wishing each other bad luck or cursing – in English-speaking countries, the expression "break a leg" replaces the phrase "good luck". The exact origin of this expression is unknown, but some of the most popular theories are the Leg Line Theory (also known as the Curtain Theory), the Shakespearean Theory (also sometimes referred to as the Traditional Theory), and the Bowing Theory.
In Australian theatrical circles saying "good luck" is also avoided, but the replacement is often "chookas!" This may be due to the belief among some dancers that saying "break a leg" may actually result in broken bones. According to one oral tradition, one of the company would check audience numbers. If there were not many in the seats, the performers would have bread to eat following the performance. If the theatre was full they could then have "chook" —Australian slang for chicken— for dinner. Therefore, if it was a full house, the performer would call out "Chook it is!", which became abbreviated to "Chookas!" It is now used by performers prior to a show regardless of the number of patrons; and may be a wish for a successful turnout.
Professional dancers do not wish each other good luck by saying "break a leg"; instead they say "Merde!", the French word for "shit". In turn, theater people have picked up this usage and may wish each other "merde", alone or in combination with "break a leg". In Spanish, the phrase is "mucha mierda", or "lots of shit". This term refers to the times when carriages would take the audience to the theatre. A quick look to the street in front of the venue would tell if the play was successful: a lot of horse dung would mean many carriages had stopped to leave spectators.
Opera singers use "Toi toi toi", an idiom used to ward off a spell or hex, often accompanied by knocking on wood. One explanation sees "toi toi toi" as the onomatopoeic rendition of spitting three times. Saliva traditionally was supposed to have demon-banishing powers and, in various cultural traditions, spitting three times over someone's head or shoulder is a gesture to ward off evil spirits. A similar-sounding expression for verbal spitting occurs in modern Hebrew as "Tfu, tfu" (here, only twice), which some say that Hebrew-speakers borrowed from Russian.
An alternate operatic good luck charm, originating from Italy, is the phrase ""in bocca al lupo!" ("In the mouth of the wolf") with the response "Crepi il lupo!" ("May the wolf die"") (see "Standard Dictionary of Folklore, Myth & Legend").
Ghost light.
One should always leave a light burning in an empty theatre.
Though it is a superstition, it does have practical value as well: the backstage area of a theatre tends to be cluttered with props and other objects, so someone who enters a completely darkened space is liable to be injured while hunting for a light switch.
Ghosts in Broadway Theatres.
In 2005, "Playbill" ran an article about Broadway theatres that were believed to be haunted. The following is a list of hauntings from that article:
Whistling.
Related to a similar rule for sailing ships, it is considered bad luck for an actor to whistle on or off stage. As original stage crews were hired from ships in port (theatrical rigging has its origins in sailing rigging), sailors, and by extension theatrical riggers, used coded whistles to cue scene changes. Actors who whistled could confuse them into changing the set or scenery at the wrong time and this could result in injury or death, especially if they were flying set or backdrops in or out. In today's theatres, the stage crew normally uses an intercom or cue light system.
Shoes on a table.
It is considered bad luck to place one's shoes on a table.