Research

Once a script is in place and everyone has a general idea of the direction in which they are heading the next stage is to start researching things. Naturally in the sound production side of it, people are going to start looking into the noises they're going to need, the items that are capable of producing those, the equipment and software needed to record those noises  voice actors suitable for the roles etc... There are numerous areas that have to be carefully looked into before production can even be considered. Looking into voice actors is a hugely important part of the process, if you have the wrong person for the job, the script can go from being a masterpiece to becoming a mumbled mess. The link below gives a great example of how the use of Ashley Burch in Borderlands 2 took what may have been a slightly odd choice of words and made them into something believable that really worked in the context of the game. 
http://www.quartertothree.com/fp/2012/09/23/getting-voice-actors-right-in-borderlands-2/
The video below is a great example of some of the most iconic voice acting used in animation, the cast of Toy Story is made up of iconic and renowned actors however they really take on the characters they're playing, if you close your eyes it isn't Tom Hanks that you picture stood there saying the lines, it's Woody acting out the scene. 

It isn't just the actors that need to be searched for, it's trying to find the right set of noises that will slot hand in hand with the aesthetic of the animation. Disney animations for example wouldn't be the same without the grand orchestral pieces playing throughout, where as more modern animations usually prefer realistic sounds to match with the animation. It's vital to experiment and play around with sound at this stage to get exactly what you need at later ones.