Hiring voice talent is mostly a briefing problem. The clearer the brief, the fewer rounds of revisions you pay for.
Write the brief before you look
Include the script or a representative extract, the audience, where it will be used, the length, the tone in three adjectives, a reference you like and a reference you dislike. Say whether you want a natural conversational read or something more performed. Give pronunciation for names, brands and technical terms.
Casting
Ask for a short custom audition using your actual copy rather than a generic showreel. Reels are edited and mastered; a raw read tells you far more. Listen on the same kind of speakers your audience will use. Judge intelligibility and warmth before you judge technique.
Usage, not just time
Voiceover pricing has two parts: the session fee and the usage. Usage depends on where the recording runs, for how long, and in which territories. Internal training material, a podcast advert and a national television campaign are completely different prices for the same hour of work. Agree usage in writing and say clearly if you want a buyout.
Technical requirements
Most professional voice artists record in their own treated space and deliver finished files. Specify the format, sample rate, whether you want processed or raw, whether takes should be split into separate files, and whether you need each line as its own file. Ask for a backup of the raw session to be kept for a period.
Directing a session
If you are directing live, give direction in intent rather than technique. Say the character is reassuring a nervous customer rather than asking for a warmer tone. Read the line yourself if words fail you. Get one clean pass of the whole script before you start perfecting individual sentences, so you always have something complete.
Revisions
Agree how many pickups are included and what counts as a revision. Script changes are not the same as a re-read of the same words, and most artists charge for the first and not the second.
Building a roster
Keep the details of voices that work and the projects they suited. A small trusted roster removes the casting round entirely for future work.