A Cover Letter should be a personal letter (not general) that addresses the director, agent, manager, or casting director, in order to tell him in a few more words why a specific actor is right for a certain job / role.
Your cover letter really depends of the purpose of your submission. If it is to a casting director you may point to a specific skill that might get you the part. If you are writing to an agent, then mention that you're seeking a new representation.
An actor's cover letter is extremely important. Some actors don't bother sending one because they assume the casting director don't have time to read it. It is true that sometimes a casting director is under a lot of pressure, therefore is short of time, but if you bother to write a cover letter, they will bother to read it.
Our advice is to always bother writing a cover letter. It shows that you take your career seriously.
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