In most theatrical settings, open flames are prohibited according to strict fire regulations. You can, however, create the effect of fire by a variety of means, including stage lighting, fake flames, blowing strips of fabric and projecting images. To select the appropriate effect, consider the tone of the performance and the mood you intend to create onstage.
Offstage Lighting Effect
Create the effect of a huge, out-of-control fire burning offstage by setting up indirect stage lighting in the stage right or leftwings. Set up a light with a wheel that rotates different coloured gels -- red, orange and yellow -- in front of the light, changing the colour that is thrown on stage as it revolves. The changing colours, when reflected on the set and the actors onstage, will give the effect of flames burning offstage. Add the sound effect of a crackling fire to augment the sense of actual flames burning.
If you need flames for a domestic set, one depicting a family living room or a home in a period play, consider using a fake fireplace you can purchase as a whole piece, including the mantel. This only needs to be plugged in to give the effect of a cosy hearth fire. Alternately, consult the company Back Stage Technologies for information on its trademarked product called "Fake Flame," which produces an actual three-dimensional "flame" that is completely safe and is not actual fire. This Fake Flame could add ambience to a show depicting spooky scenes or to give torches a realistic flame for medieval or Shakespeare plays.
Old School Flame Effect
The Victorians used some imaginative renderings of flames for their melodramatic extravaganzas of the 19th century. Consider recreating their stage effect for "flames" by cutting strips of red, orange and yellow cloth or crepe paper into long strips. Fix these to a hidden location on the set, such as the upstage side of a window or doorway. On the cue for the flame to appear, a hidden fan blows the strips of colour up in the air so that they seem to be coming from the window or door. This effect, when repeated in each window and door of the set, can make a room appear to be on fire.
Projected Flame Effect
Twenty-first century technology makes it simple, in most cases, to simply project an image onto a stage of any effect you desire. Record video footage of actual flames and project them onto an upstage cyclorama (the white curtain that hangs at the back of many stages) or directly onto the set. Simply mount the projector so that the image hits the stage in the way that you prefer. This effect can be completely operated by computer and be accompanied by sound effects that add to the sense of a burning fire. A scene in which the set or characters are to be engulfed in flames would work well with this effect. Use projected flames to highlight a monologue about fire.