Theatre Organ
When movies were silent they were usually accompanied by a piano in a smaller theater, and a few larger theaters would use a small pit orchestra. The movie studios would often send sheet music with suggested themes for the various characters and certain scenes for that film. The instrumental accompaniment added the emotional element, but there were no sound effects to add the realism for the actions on screen.
In a very few theaters, people either in the pit or behind the screen would play drums and percussion, and use other props to add the sound effects live during every screening.
Robert Hope-Jones, an Englishman living in the United States had the idea of incorporating some new features into an instrument, which he initially called, a "unit orchestra". He worked with the Wurlitzer Company to make the first theatre organs, (also called cinema organs) to add both the music and the effects to films in theaters. They differed in several ways from the conventional church or concert organs.
First, instead of drawknobs that had to be pushed or pulled to enable a particular rank of pipes, they had tabs in a horseshoe arrangement that could quickly flipped on or off.
Second, theatre organs had instruments not found on traditional church organs like drums, mallet percussion, tuned sleigh bells, chimes, other percussion, and even a piano, that could all be played from the organ's keyboard. In addition there were also sound effects like boat and bird whistles, car horns, sirens, and a cylinder with materials that sounded like rain or the ocean when it was rotated.
Third, these organs made great use of tremulants, which were devices that created a vibrato effect by mechanically varying the air speed.
Until most films had sound in the late 1920s, theater organs were a vital part of the movie experience.