Confirm your shooting location, the lighting conditions and the time of day you will shoot the movie. These are essential when choosing the type of film to use to avoid grainy footage; the light sensitivity of the film greatly affects the chances of producing grains on the image.
Decide what kind of film to use. Choices range from the slowest to the fastest film stocks. This measure is based on film speed, which is referred to as the film's ASA (American Standards Association or ISO (International Organization for Standardization) number. Some of the popularly used film speeds are ASA 100, 250, 320, 500 and 800. The faster the film, the higher the number, and the more light sensitive it is. The higher ASA numbers are also more prone to graininess, but they are more light sensitive and are best used when shooting in low light. However, underexposure using a fast-speed film would cause even more graininess.
When using a slow film, the grain can also appear if you don't have enough light to properly expose the image. It may not appear as relatively big as the grains on a fast film, but the overall quality and details of the image still greatly suffers. This is because the film is not able to get enough light to prevent underexposure of the image.
Use enough lights in your shooting set up. Low light causes the image to become more grainy due to underexposure. The poorer the lighting, the more grain can appear on the image.
Process the exposed film properly. Use the right film developer meant for the kind of film used. Also, avoid over development as this brings out more grains on the film as well.