What is the character's background? What role does this character play in the story? Have you written how the character's voice and personality should sound like?
How many lines of dialogue will be required? How many successful takes are you expecting from each line? Has all the dialogue been written already, or are you expecting more dialogue later in the project?
Remember that involving anyone under the age of 18 requires guardian approval for participation; and if NSFW material is going to be even slightly in your game, you won't be allowed to have anyone under 18 involved in the project at all. Will you be needing participants to forfeit controls over how their voicework would be used? Do you have a sunset clause to gracefully delete (original and any copies of) recordings after a certain date?