“Break a leg” is a phrase that means “good luck.” It’s typically said to actors before they go on stage for a performance, particularly on opening night. The expression probably reflects a superstition (perhaps a theatrical superstition) in which directly wishing a person "good luck" would be considered bad luck, therefore an alternative way of wishing luck was developed.

The phrase likely comes from the process of the performer breaking the leg line on the side of the stage. The edge of a stage just beyond the vantage point of the audience forms a line, imaginary or actually marked, that can be referred to as the "leg line," which is named after a type of concealing stage curtain: a leg.

For an unpaid stand-by performer to cross or "break" this line would mean that the performer was getting an opportunity to go onstage and be paid; therefore, "break a leg" might have shifted from a specific hope for this outcome to a general hope for any performer's good fortune. There are a couple of credible theories on the origin of this superstition. The first comes from Elizabethan England when actors were sometimes thrown money after a good performance. They would kneel down to pick up the coins, “breaking” the straight line of their legs. Another theory comes from vaudeville performance when there were multiple acts scheduled. Masking curtains on the sides of the stage are called legs. If a vaudeville actor were to “break a leg,” they would cross in front of the legs onto stage to perform that evening and get paid for their work.

Even less plausible, the saying could originally express the hope that an enthusiastic audience repeatedly calls for further bows or encores or that they bang their chair so strongly that it might "Break a (chair) leg".

Saying Macbeth in a theatre?

Most actors will only refer to Shakespeare’s famous tragedy as “The Scottish Play,” “The Bard’s Play” or even “Mac B.” Some believe that Shakespeare gave his three witch characters in the opening scene a real spell incarnation for the lines spoken. In retaliation, a coven of actual witches then cursed the play because Shakespeare had no permission to use their sacred words.

It is said that the actor playing Lady Macbeth tragically died on opening night in 1606 and Shakespeare himself had to step in to continue the performance. Dueling Macbeth productions in New York caused the great Astor Place Riot in 1849, leaving at least 25 dead and hundreds injured. Other productions have been plagued with accidents, including actors falling off the stage, mysterious deaths, and even narrow misses by falling stage weights, as happened to Laurence Olivier at the Old Vic in 1937.

So, don’t tempt the witches, and don’t say Macbeth in the theatre!

Whistling is bad luck...

Whistling in a theatre is considered to bring bad luck to a performance. This superstition started in the middle of the 1600s when theatrical scenery began to fly up into the towers, or be brought in to the stage from above.

Sailors had extensive knowledge of ropes, rigging and knots and were hired backstage as run crew as they had that experience working on the tallships. They were keenly aware of how to tie knots securely, work together in unison to handle heavy scenery, and would work in theatre during when they were not working on a sailing ship.

Like on a ship, the sailors would communicate with each other through whistles to bring a backdrop in or out. Consequently, if an actor were to whistle onstage it could signal the start of a scene change early or they could get knocked out from incoming scenery. Luckily, today we communicate through headsets and que (cue) lights in modern theatre, but the tradition of no whistling still remains.

A drop shouldn't drop – A drop curtain is an unframed curtain that is lowered to the stage from above, often serving as background scenery. It is raised and lowered vertically, rather than drawn to the side.

A block and fall neither blocks nor falls – Block and fall refers to a theater's extensive pulley system that is used to raise and lower lights, scenic pieces, and sometimes even actors during a production. It can also be called a 'fly system.' It consists of two or more pulleys with a rope or cable threaded between them, and can lift or pull heavy loads.

A prop doesn't prop – A prop is any object used on stage by an actor during a performance that is portable. It is considered distinct from the scenery, costumes and electrical equipment, and is the commonly shortened version of 'a property.'

A cove has no water - A cove is a position in the theater where stage lighting can be placed and hidden from the audience.

A running crew rarely moves anywhere – A running crew refers to the members of the technical crew who supervise and operate (or "run") the various aspects of the production during the time of performance (i.e. not set-up or take-down.)

A purchase line will buy you nothing – A purchase line is part of the block and fall system. It is the rope that the operator uses to move the scenery or lighting unit up and down during the performance.

A trap won't catch anything - A trap is an opening in the stage floor, which while normally covered, can be opened and used for special effects, such as having scenery or performers rise from below.

A green room isn't green – A green room is the traditional name for the room in which actors gather to wait for their entrances. It is thought that the original room used for this purpose at London's Drury Lane Theater was actually painted green, and the name has stuck around.