Minor addition to the writing question list.
Also how is it like to write without having any idea how the character will sound?
When writing, most designers envision how the character sounds as they're writing - when the time comes for auditions, they provide a series of sample lines, a picture of the character in-game, breakdowns of the age, brief history, etc., and then the casting agency will run through auditions looking for someone who can deliver the lines as envisioned. I was happy with the auditions Blindlight delivered for New Vegas, and I thought the companion actors they brought into the studio for the characters I wrote, while not big names, did a great job - a lot of it is in how Blindlight handles the auditions, and so much of a character is in the delivery, that if you can get the casting right, it just makes the process go more smoothly.
If you're fortunate and the schedule works in your favor, you can also request a specific voice actor. This depends on timing and cost, and as a general rule, the more famous an actor is, the less flexible the time in the studio and less availability for pick-ups (the equivalent of Voice-Over bugfixing if a level quest changes, a character's line is missing, or we need to add a line to fix a missing sequence).
Other times, you're told who the voice actor is first, which is rare for me. When that happens, you watch everything you can featuring that actor and try to write to the actor's strengths. As an example, for Fallout: New Vegas, John Gonzalez studied John Doman's acting when writing and Eric Fenstermaker did research on Felicia Day to get the tone of the characters that played to the strength of the actors.