There are many ways to look at that intersection. Some classical composers also wrote for film. Some film composers wrote classical works for the concert hall. Some classical music has become famous primarily because of its use in a film, and some film scores have been expanded into classical works.
Here were my selections for week one of #ClassicaDay #ClassicalFilmScore. I only post on the week days, and there were only two in the first week.)
Richard Addinsell (1904-1977) - Warsaw Concerto
The "Warsaw Concerto" was composed for the 1941 film "Dangerous Moonlight." The story involves a shell-shocked combat pilot who was a piano virtuoso before the war. In the movie, the concerto's written during the Invasion of Poland (hence the name). The producers wanted Sergei Rachmaninoff to provide the music, but he declined. British composer Richard Addinsell provided the over the top score that became popular as a concert work.
(Dangerous Moonlight, 1941)
Franz Liszt (1811-1886) - Les préludes
The 1854 work, Les préludes, was Franz Liszt's third tone poem. It was used in the opening credits (and some action scenes) in the 1936 Universal movie serial "Flash Gordon." Although the Flash Gordon serials enjoyed three times the normal serial budget, much of the soundtrack was culled from other sources. The score includes cues from the Universal music production library, Wagner's "Parsifal," and other classical works.
("Flash Gordon" 1936)
#ClassicalFilmScore Week 2
#ClassicalFilmScore Week 3
#ClassicalFilmScore Week 4