2012 blogpost about "chain of title" is a great introduction to the documents filmmakers will need when it comes time to make a deal with a distributor.
As Steven Beer explains, the chain of title is the "“chain” of documents that demonstrates your ownership rights in your film."
"Anyone who has an interest in financially supporting your creative work must be confident that not only do you personally represent and warrant that the appropriate rights have been secured, but that you are willing and able to demonstrate this condition via your chain of title documents."
As Steven Beer notes: "The number and array of documents to include in a chain of title package varies depending on the size and complexity of the production."
Copyright registration is typically only one piece of the package of important documents you'll need to demonstrate the ownership and permissions necessary to commercially exploit a film. Steven Beer (and Marc Pellegrino, a Cardoza Law School 3L, who helped with Steven Beer's post) helpfully list many of the documents that distributors might expect to see.
Although it's just an introduction, written in language that indie filmmakers can understand, Steven Beer's post should help explain why even microbudget filmmakers will need an attorney. If this article doesn't convince you - take a look at the delivery items list in a typical film distribution agreement. The documentation that a filmmaker must provide, before a distributor will handle their film, generally requires that an indie filmmaker seek the help of an experienced entertainment lawyer.