The essential difference between documentary and feature film is that documentaries record facts and feature films are fiction. A documentary should be an accurate depiction of real events, people, volatile issues, emotions and reactions or conditions. Feature films may aim for the same inspirational resonance and presentation of truth as documentaries, but features have no mandate to stick to the facts. Feature films invent scenes and characters and might just as readily be conceived as pure entertainment.
Movies and Motivation
A feature film is an escape, a fantasy, a comedic or dramatic entertainment that might be an expression of art and is firmly in the camp of the commercial. The desire to make art may or may not drive the production, but the film is created to be sold, to make money. The motivation for a documentary is to educate or inspire. The viewer might be convinced to share the filmmaker's outrage over an injustice, such as a genocide, or a major threat, such as climate change. The information might celebrate a marvel -- the flight of the monarch butterfly -- or chronicle an event -- preparation for a manned space flight. The conviction of the documentary maker is the incentive for making the film and it may serve as a broader call to action regarding the issue chronicled.
Fiction vs. Reality
Director Martin Scorsese thinks narrative feature films are harder to make; he feels its like putting together a demanding puzzle, because of the coordination needed for actors, crew, specialists, equipment, locations and every other element of the project. Some documentaries are shot as they happen, with the script sometimes being written after the filming, making it a bit problematic. Life must be neatly fitted into the dramatic storytelling form and it doesn't always cooperate. The bigger logistical challenge is the feature film but the greater degree of uncertainty comes during the documentary film-making process.
Fiction and the Feature Film
A feature film is a story told in sound and pictures and, even when it is based on or inspired by reality, it is fiction. The film typically follows a three-act structure with the introduction of the characters and conflict in the first act, the complications and raising of the stakes in the middle or second act and the dramatic conclusion or resolution in the third act. The story is structured around sequences and the sequences are made of scenes, with attention to plot and pacing that hold the attention of the audience. A script will be styled to convey a feeling -- the cinematographer, sometimes called the director of photography works on lighting and the color palette for the camera. Scenery and locations are painstakingly selected or created, while the costumes, music, props and casting of the actors are not left to chance. A feature film is a carefully controlled artifice, start to finish, and requires a suspension of disbelief on the part of the audience.
Hallmarks of a Documentary
The strongest documentaries have some elements in common. Each story is different but the right elements will produce an identifiable audience reaction. A great documentary may inspire powerful emotion or a change in behavior, shock or surprise the audience, present information and challenge the viewer to form an opinion about it, espouse a deeply held belief and capture a clear, sometimes controversial point of view. There is no rigidly defined technique -- documentary films employ many of the methods used in making commercial feature films and may include animation, studio shots with sound and lighting set up for an interview, cross-fading, multiple camera angles and scene-setting beauty shots. But technique exists in service to the portrayal of reality. A true documentary will sacrifice artistic effect in favor of authenticity.