I wanted to post this in Tips and Tricks, but apparently I can't create new threads there, so...
This was brought to you by the lovely Haushinka.
The casting process can be intimidating the first time or two around, so here are some tips to make sure you're happy with your cast and the end production. Producers, feel free to add anything you like to keep in mind that you find works when casting!
Don't cast voice actors with bad microphone quality.
Seriously, just don't. Even one muffled, tinny, or maxed-out recording brings the quality of your ENTIRE production down and makes it sound completely amateur. They don't have to be studio-recorded, but anything with lots of clipping, crazy static that you can't get rid of, etc...it's just not worth it. Same for pops and other noises---if they're in the auditions or the demo, they're going to be in the lines you receive and they're pretty hard to get out. If you really like the person's voice, let them know you'd love to keep them in mind for future productions when they get access to better recording equipment.
Always be open to hearing new voices.
When you've produced for a while, it becomes very easy to have a "go-to" cast that you use for everything, and we've all been guilty of this at some point. It's easy to see why: you know these people are dependable, and you want to show your appreciation by offering them more work. Completely fine. BUT, every so often, test the waters by holding an open audition or listening to some demos when you've got a character you can't quite seem to place. The person you find may very well become a new regular.
Popular names are great, but don't use that as the only reason for casting someone.
Well-known voice actors will bring a bit of extra recognition to your project, but are you sure they're the right fit for your character? Also, keep in mind that they often get a lot of voice requests and may take a while doing lines for you. If you're not sure, try offering them a side or cameo role. And when you first listen to auditions, try not to pay too much attention to whatever screen name is attached to it---keep a window with the character's picture open and listen to what really embodies your vision for it.
Consider general reputation/experience when casting someone.
Using new talent is awesome, but if someone hasn't had any real experience, it might be better to start them off with a smaller role and move them up to a lead in your next project so they can get a feel for what they're doing. Similarly, if a voice actor you cast has a bad habit of ALWAYS being late with lines or dropping out of lots of productions, don't waste your time continuing to cast someone who has not been dependable to you on multiple occasions.
Choose wisely which roles you give to people you know.
When finding out you produce, it's natural that your close friend, girlfriend, brother, etc. will want to try their hand at voice acting, and it's also natural that you will want to give them a part. Unfortunately...chances are, they can't act. So instead of casting them as the title character, ask them if they'd like to start off with a few lines as a more minor role, and direct them through it. In most cases they will be happy just to have a chance at being part of your project and trying something new.
If you don't find quite what you want...WAIT.
Anyone who's been through the casting progress has more than likely experienced both of these situations: You find an amazing voice that just CLICKS with the character right away, and then you've got a character with a myriad of mediocre auditions but nothing that really stands out as being just right. It's normal to be impatient and want to just pick something that sort of works so that you can get started, but you'd be surprised what you can get by waiting an extra week and doing a little poking around.
Never be afraid to ask for redoes.
Whether in an audition or the actual recording process, if you're just not feeling a take that your VA sent you, ask for a different take, but always be specific. Give them an audio file for reference if you need to (just keep it concise and don't ramble on for ten minutes!) You're the director---it's your job to get what you want out of your talent. But be realistic and don't cast someone who simply cannot act in their audition, because although you can direct them to an extent, chances are their acting will still be poor in the final production.
Make your audition thread inviting and professional.
It will take you a few extra minutes, but go for it. Put up pictures, neat summaries, spell check and proofread. Give your project HYPE. Don't be afraid to mention awards you've won on NG or amount of YouTube views you've had---voice actors will jump at the chance to be part of something big. The readability factor is so important because many established voice actors have become more picky about what they audition for and will skip over something that doesn't really catch their eye.
Show appreciation to your voice actors by inviting them back.
If you like what they've done and they've been dependable, keep their contact information on file and let them know if a character arises in the future that they would fit. Generally, the more you work with each other the more you will understand each others' styles and mesh well.
Ask a friend to listen in when you're weighing the top two or three contenders for a role.
When you're having doubts about who to cast, it really helps to have a fresh set of ears. It doesn't even have to be an experienced producer---any person can tell you whether something sounds good to them or not. If you're still not sure, feel free to do "callbacks" where you ask for additional takes or lines from the people you're choosing between.
It really is OK to use your own voice, just be smart about it!
Some producers feel like they have to be a sort of martyr by refusing to cast themselves even if they really want to play a role. The fact is, you're the one doing all the mundane behind-the-scenes work, and it's your production. You can play whomever you want and you don't have to feel guilty about it. If it's an original script that you wrote... you as the author know the character better than anyone else. Do keep in mind, though, that you have to live with the results, and you don't want to be the one bringing your entire production down if your quality and/or acting skills are not up to par. Choose a character that you can realistically pull off (it's okay to challenge yourself) and just beware of casting yourself as multiple characters unless you can really differentiate the voices.
There is no harm in inviting someone to audition for your project.
If you think a certain person would be great at playing a particular character, feel free to ask them to audition if they have the time. Even if they don't or they're not interested, there is no harm in asking! Many people do not regularly check the auditions section and are glad to see that someone invited them to try out. But please, make sure to personalize your invitation---nobody likes a generic, spammy mass PM saying "hey voice actors, please audition for this!"
Don't promise more than you can put out.
If you want to keep great people auditioning for you, fulfill their expectations. Meaning, be realistic about what you have to offer. I have seen so many producers say things like "This project is no pay for now, but we're trying to get it on the Xbox 360 and we're pitching it to Adult Swim and if so you guys are gonna get paid!" Almost every time, nothing of the sort ever happened. Anyone can send their Flash animation to TV networks, but if they don't actually look at it, what is the point in getting people's hopes up? Wait until you have something concrete. It's fine if it's a student project for no pay; just be upfront about it rather than saying you're trying to get it made into an actual video game (a time consuming and expensive process that is more difficult than you think; do your research first.) The same goes for making people do hundreds of lines for something that looks promising but it turns out you don't have animators, etc. We all want to hype our projects, but just be realistic---those of us who have been here long enough start to recognize what pretty much amounts to a scam.
Finish what you start.
This ties in with the above, but if you develop a good track record of completing and releasing your projects, people will be attracted to working with you because they know it will come to fruition. Ensuring you finish what you start is as simple as, "don't get in over your head". If you want to do a fandub, start with a short scene rather than a full episode so you know what you're getting into. If you want to make a Flash animation, learn basic animation first. If you want to make a radioplay, get familiar with your audio editing program and how to level volumes, etc. If you're busy with school, work on your project slowly as you have time rather than cancelling it. No one will hold one cancelled project against you, but make it a habit and you'll have a harder time getting and keeping good talent.