The basic rule is 70 years after the author's death, not after creation or publication. In addition, works by corporations have different terms, but it's always going to be more than 70 years about publication, at least according to current law.

To calculate when Casablanca specifically would enter the public domain would mean cross-referencing several laws, which I'll leave to someone else since it's all too likely there's an easy online tool that I don't know about.