Unlike literary fiction, which is typically based on characters, situations and events that are entirely imaginary, cinema always has a real referent, called the "pro-filmic," which encompasses everything existing and done in front of the camera.
Since the emergence of classical Hollywood style in the early 20th century, narrative, usually in the form of the feature film, has held dominance in commercial cinema and has become popularly synonymous with "the movies." Classical, invisible filmmaking (what is often called "realist" fiction) is central to this popular definition. This key element of this invisible filmmaking lies in continuity editing. In this style, narrative and characters are foregrounded, helping the audience to lose themselves in the unfolding fiction.
Prior to this, movies would include a notice at the beginning informing the audience that the events portrayed were fictional and that, if they happened to have any similarity with real events, this was "purely coincidential". Now a days, films tend to include a notice when they're not portraying fictional events, which may be "based on a true story" or some variant thereof.