Your voice actor will be able to offer great ideas about their characters, but here's advice on how to use them to their full ability.
A voice actor needs a few things to give their best performance:
You might also want to consider that their voice will have a different quality in the morning, afternoon or evening. It is best (as with most things) to leave the voice actor plenty of time to record so that they can use their voice when it is at its best. Less experienced voice actors may also feel nervous - do your best to relax them so they can record at their best. If the sound designer is recording them in their studio, the sound designer should always keep some water handy for the voice actor and bear in mind the timing of the session.
A good voice actor will often know how to use and process their voice to sound its best. It is a good idea for a sound designer to ask what processing a voice actor has put on their lines. Voice actors should work closely with your sound engineer, especially if more unusual results are needed. Those underwater zombie robot chipmunk sounds you needed might take a little collaboration to get right.
Most voice actors will record multiple takes, often including two different options for each line. In between each take will be a moment of silence, or sometimes a clap to make it clear that they are separate lines or takes.
These recordings can then be passed on to your audio editor (generally just the sound engineer) who can chop up the lines into usuable fragments, ready to be synced with whatever you want. Your audio editor will have a lot of decisions to make and is sometimes left to decide what take to use, so be sure that they are also filled in the story and backstory of the character so that they can make informed decisions.
Otherwise, you can try out editing the lines yourself! It actually is not too difficult to chop up the lines for yourself, though cleaning up imperfections and adding in processing is best left to an experienced sound engineer.
Of course, Echo Warriors can handle this entire process for you and guarantee beautiful results!