A formal cover letter may or may not be needed when submitting your package to an agent or anyone else. If they have a website and allow unsolicited submissions, they'll usually be pretty clear about what their process is. That being said, it's always a good idea to be ready to include a cover letter in your package. It lets them know why you’re sending your package and it gives you a place to include anything about yourself that you wouldn’t generally include on a resume. Note: The cover letter might simply be the information you put in the body of your email.
Much like a resume, a cover letter has differences between one you would send when applying to be a lawyer and the one you would send to find an agent.
Here are some notes when making a cover letter.
Address the letter to...
...a specific agent within the agency. Don't write "To whom it may concern." That's too impersonal. However, just because you're addressing it to one agent, doesn't mean you need to send every agent in the agency a package. They will generally pass your package around to all the agents to see if any are interested, and if none are, then it would go in the trash.
Keep it short
Your letter should take less than 30 seconds to read. Agents are extremely busy and they don’t have time to read an essay on who you are and why you want an agent.
You could include a line or two or something that has happened recently or will happen soon. Some examples would be if you recently graduated from a full time acting program; you were performing on stage in a show (preferably with good reviews); you will be performing on stage in the near future.
Just like your resume, you need to include at least your phone number and email. You never know who may get a hold of your resume. If you include an email address, make sure it’s clean and appropriate.
The resume has a format set up to show information in a very clean and easy to read manner. Even though a cover letter needs to be short and to the point, there's always room to put your personality into it. A generic cover letter with the right words is good, but a cover letter with a little flare is better.
Always double-check your letter and resume! Spelling is very important. Once again it's the attention to detail. Proofread anything you're going to send out. Make sure every word and name is spelt correctly. It's worst when you spell the agent or agency name wrong. It happens. You want them to look at the information you're providing them, not any spelling or grammar errors you may have. There's no excuse for it with programs with automatic spell check. Have someone else look it over for you. That way they could catch anything that you missed.
One of the most important things to do is make sure everything you send has your name on it. This includes the cover letter, resume, demo reel and headshot. In the digital world, that means the file name should have your name on it. This ensures that if the pages get mixed up or lost they will at least know which picture belongs to which resume.