To what degree you want to participate in the recording session will impact cost and voice choice. Generally the widest choice of voices and most cost effective method is "Over The Fence".

"Over The Fence"
In this scenario you pass over the script with some written guidelines on style and receive the finished recordings by email mp3, wav etc. To ensure you get the style you want, most clients will refer to a sample on the voice artists demo - or indeed if it's a large script we can often record the first paragraph or so and get you to listen and approve before the full script is recorded.

"The Listen-In Session"
With a listen-in session you can listen into the recording session via telephone/Skype. You don't need to hire or visit a separate studio, just need access to a telephone and sit in a quiet location.
An engineer/director will record and direct the bulk of the session and you can interject with comments.
Due to the nature of these sessions they are costed up based on time rather than script length - and given the script we can advise how long the session will need to be. Not all voice artists can be patched in live, especially our foreign language voices, so this method may not be available for your chosen voice artist.

"The Directed session"
In this situation it would be your responsibility to record and direct the session, and remove errors from the resulting recordings. You'd use a recording studio which has the technical means to link up to the voice artist who is present at another studio.
This method is normally costed up in hour session blocks, and you'll normally find studio hire will be by the hour too. You'll have much more control of the session recording but the downside is that it will probably work out more costly, and your selection of voices may be more limited. Also if you wish to do 'pickups' or alterations after the session completes, perhaps you tweak the script, then this can work out costly.

"Voiceover In-Person"
The traditional way of recording a voiceover where the artist will travel to your designated studio and you record and direct them in-person.
This is ideal if you want to make an 'event' of the recording, perhaps you have a client who you want to impress and invite so they are hands-on throughout the creative process.
Ideal if your chosen voice is based in a major city such as London and you are happy to arrange studio hire.
Nowadays many professional voice artists are not based in London, although some are happy to travel in for a session (but you may incur additional expenses).