For those of you unfamiliar with ACX's process, the author puts his book up on the site with details and a brief audition script. (I used a conversation between Ric and Caz, as it's vital they sound right.) Actors, known as producers, who are interested can make a recording of the script and send it to you. Also, you can choose producers whose samples you like the sound of and ask them to audition. It's a good idea to go to Audible, look up the narrator, and if she has recorded audio books, check out her reviews.
You can pay the producer an agreed amount per finished hour, or offer a royalty-split deal. If your book is currently selling well, ACX may offer a stipend to encourage producers to audition.
It's interesting, hearing an actress read your lines. I have a very, very clear idea of what my characters sound like, and would find it impossible to accept anything much different from my conception. It's essential the actress is able to read intelligently, or she will stress the wrong word and change the meaning of a sentence. On the other hand, a good narrator will surprise you with a slightly different interpretation which you may quite like.