Trapdoors are hidden openings in the stage floor, and have been used in theaters for centuries to enable actors to seem to appear and disappear instantaneously. When used intelligently with a little misdirection or smoke and mirrors, a trapdoor allows an actor to quickly drop down beneath the stage, disappearing from view, resulting in an extremely dramatic effect.
Harnesses and wires attached to the actors from pulleys and devices above the stage can allow the actors to appear to leap and fly. The use of thin yet powerful wires or cables allows them to fade into the background, contributing to the illusion that the performer is truly defying gravity.
To simulate blood in stage violence, bladders and pouches of stage blood can be taped to the actor's body, and burst by the actor in secret at the appropriate time to simulate bloodshed. For a suddenly bloody mouth or facial injury, an actor may use a "squib," a small capsule or balloon filled with fake blood that is bitten, and breaks, on cue. Stage blood is typically corn syrup mixed with a red dye from a theatrical effects supplier--the dye doesn't tend to stain as easily or permanently as food coloring.
The element of sound, when creatively used, can be a powerful and useful special effect. For centuries, sheets of metal have been shaken backstage to simulate thunder, while firecrackers, clashing metal pans, and more have helped to create the sound of battle. In today's world of computers, it is simple for any sound technician to offer highly realistic sound effects for everything from storms and seas, to battles, birdsong, traffic, factories, and a thousand others.
It's also surprisingly easy to create a variety of believable special effects onstage, just through the simple and creative use of light. You might use strong yellow light gels to create the illusion of sunlight, for instance, or use blue or pale silver colors to simulate moonlight. Other light colors can help to suggest such locations as Heaven, Hell, magical kingdoms, far-off planets, and everything in between. Flickering red lights can help to simulate a campfire or fireplace, while colorful or flashing lights might simulate a computer or ship's navigation panel, while strobe lights can give the illusion of slow motion and more.
With today's sophisticated technology, it's easy to use projectors against walls, scrim or curtains to provide stunningly realistic water or other effects. Meanwhile, some popular effects remain as simple as they were years ago, as with the simple, tiny dancing spotlight which has been used for centuries to create the illusion of a fairy flitting across the stage, delighting children for centuries.
Several beautiful weather effects are surprisingly easy to achieve through the simple use of a bag, drum or tumbler suspended high over the stage, and filled with confetti. When the drum is turned, or the bag is shaken, confetti falls gently and regularly from the opening onto the scene below. Soft, white pieces of confetti can be use to simulate snow, while shiny pieces might simulate rain or a magical spell.
When combined with just a touch of water, dry ice can be used to create billowing and atmospheric clouds of smoke that cover the stage floor to simulate a misty or ghostly scene. For larger concerts or stage productions, dry ice machines can be used to generate a specific flow of mist or fog.
When used by skilled and experienced crew members, pyrotechnics incorporating fire or smoke can be used to create powerful special effects. Flash powder is a common theatrical element that can be used to create the sudden puff of smoke onstage, while flash paper is an easy way to create a sudden small flash of flame.