How to know if it's best to write an email or letter when applying for acting jobs

Firstly, do your research into who you're writing to. For example, if you're applying for a specific role/casting breakdown it'll state in the breakdown what kind of communication the casting director would prefer. It's always really important to take the time to thoroughly read the breakdown before applying, as sometimes they ask you to email or write to the company or casting director rather then apply through the casting website you've seen the breakdown on.

And vice versa, if you've spoken to a casting director via email before, but you're asked to apply via the casting website for a particular role, don't think that by emailing the casting director directly you'll cut out the middle man and have more of a shot at the role. There'll be a reason why they're only taking submissions via the casting website, and you risk annoying the casting director by going against this. If anything, mention your previous communication with that casting director in the notes/covering letter section of the website application.

If you've not seen the role on a casting website, and are instead writing to a specific theatre/production company or freelance casting director, again check the "Casting Policy" section of that theatre or production company's website. Obviously these kind of organisations receive thousands of applications a year, so have strict criteria on how to apply for work. See http://www.octagonbolton.co.uk/casting-policy for an example of this.

The same goes if you're applying to a freelance casting director. Always check to see if they have a website, and if they do whether they tell you how they prefer to be contacted. If all fails, I'd chose to email them, as it's greener, and it's the preferred method of communication for most industry people now.

When you do contact someone though, don't send a follow-up letter/email, and don't send the same email you've already written but in a different form of communication (e.g. in a letter) in the hope that the person you're writing to will have to reply if you bombard them. Firstly, it's annoying to receive more then one email saying the same thing, and secondly if you don't hear back from your casting inquiry that's because you've been unsuccessful. It doesn't mean that if you write to that casting director in the future you will always be unsuccessful, it just means you weren't right for that particular part.

So, in short, to answer your question of, "When applying for acting jobs, is it best to write an email, letter, or both?":

1. Do your research
2. Only send one of the above. NEVER send both.
3. And be patient. Just because you don't hear back from your letter doesn't mean the casting director hasn't read it. If you really want to work for that theatre/production company, or be seen by that casting director, keep an eye out for other casting breakdowns they're advertising and apply for any that you're suitable for. And make sure when you do write that you're specific, state exactly which role you'd like to be seen for. Also, write to them when you're in something they can come see, it's good for them to see you're working.
4. And the most important rule, be professional in your letters. Don't ever be bitter, desperate, aggressive or too personal in your approach. This is why follow up emails are never a good idea, as they can often come across as one or all of the above.