In general (if not always), films are about some theme or topic, and fictional films involve characters and types of characters. There are several complications in researching this topic, in part because sometimes there's a distinction between how the Library describes films vs how it describes books about those films ... and sometimes there doesn't seem to be. Please read the explanations below carefully.
These three sources are very useful, but they are of course limited by their publication dates and the number of films they cover.
AFSO identifies screenplays' subject(s) and genre, and lets you search by these criteria. Note that AFSO only includes American films. See Screenplays/Filmscripts for more information.
Encyclopedia of film themes, settings and series. Richard B. Armstrong and Mary Willems Armstrong. Jefferson, N.C.; London: McFarland, 2001.
This encyclopedia is especially useful for obtaining examples of films with themes like amnesia, deafness, genies, ice skating, mirrors, photographers, sewers, and windmills.
The Ultimate Movie Thesaurus. Christopher Case. New York: Henry Holt and Co., 1996.
Consists of two major sections. The first is a list of films by title, providing a one-sentence synopsis, its genre(s), and its topics or themes. The second section lists the genres and topics, and the films that provide examples.
Films on a specific character, character type or group are typically in the form [Character]--Drama or [Character] films. The "--Drama" subheading is the proper one, but the reason there may be two forms is that often these are considered a genre (although sometimes the subject heading is applied by mistake). See also Genres, Styles & Categories.
Films on a theme or event usually have a subject heading taking the form [Topic]--Drama, but there are also related genres (e.g., War films).
Films that are part of a series (or implied series) may receive [Series name] films. A series is usually treated as though it's a genre.
NOTE: Catalogers are not required to apply any headings of this type to films. You may need to supplement searches for films on a particular topic by looking at filmographies and similar sources.
Characters, Character Types and Groups
Topics and Themes
Series (these are usually treated as a genre)
Two Forms (try both)
Critical works, including documentaries, about a topic as seen in motion pictures. (Sometimes these headings are also applied to fiction films. This is usually an error, but it's appropriate for a film like Hollywood Shuffle, which is in fact about African-American actors in Hollywood.)
Social Issues as Shown in Films
Sex and Gender as Shown in Films
Social Groups and Character Types as Shown in Films
Characters in Films (usually treated as a genre)
The American Indian in film. Hilger, Michael. Metuchen, N.J.: Scarecrow Press, 1986.
The bent lens: a world guide to gay & lesbian film. Edited by Claire Jackson and Peter Tapp. St Kilda, Vic.: Australian Catalogue Company, 1997.
Blacks in black and white: a source book on Black films. Sampson, Henry T. Edition: 2nd ed. Metuchen, N.J.: Scarecrow Press, c1995.
The Columbia companion to American history on film: how the movies have portrayed the American past. Ed. Peter C. Rollins. New York: Columbia University Press, 2003.
The encyclopedia of ethnic groups in Hollywood. James Robert Parish. Facts on File, Inc. : New York, NY, 2003.
Encyclopedic dictionary of women in early American films, 1895-1930. Denise Lowe. New York: Haworth Press, 2005.