Use a film for which permission is not required.
Although the rules for determining the duration of protection can be complicated and may depend on facts that are simply undiscoverable without many hours of research, one bright line rule does exist: any work published in the U.S. before 1923 is in the public domain.

Many copyright owners offer their works to the public with few or no restrictions. To use these works, make sure that the owner has given explicit permission to the public and heed any restrictions that may prohibit your planned use.

Sources of information about films that are in the public domain or have been offered to the public and how to obtain them include the resources listed below.

Use a work created by the U.S. government.
Works created by the federal government are not protected by copyright. However, works commissioned by the federal government and state government works may have copyright protection.