Motion pictures get tricky because of their multimedia nature. An original silent film may be in public domain, but a version of the same with musical accompaniment may be copyrighted because of the music. Same goes with early Hollywood musicals. The film itself may not by copyrighted, but songs in it are, and therefore permission is needed to show it.
Per the Pratt website “However, just because a copyright renewal was not properly filed does not mean the film lacks copyright protection. Films are often based on books, plays, or other works that may maintain copyright. If the pre-existing work is protected, than rightly or wrongly, it has generally been determined that the derived film is also protected. In addition, films are multi-layered works that make use of songs, musical scores, and other potentially protected materials such as images of artworks and trademarks.”
In Stephen Fishman’s The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More. (you can read a good deal of it on Google books), he uses a good example of the derived film His Girl Friday (1940). While the film’s copyright was never renewed, thus entering the public domain, the original play it was based on, The Front Page (1928), has had its copyright renewed and will be protected until 2024. You would therefor have to get permission from the copyright owners of The Front Page to use His Girl Friday.
Fishman’s book also has a very informative step by step guide to help ascertain whether a specific film is in public domain.