Typically voice overs are recorded then provided to the animators to animate & properly sync up the mouths to the dialogue.  Only time it's the other way around is when re-recording a differnt language dub.  Been doing 2D long enough and know enough folks in 3D to know the process hasn't changed much since sound first came into animation way back when!!

Gerard

The GR00VY GH0ULIE!

You are pure, you are snow
We are the useless sluts that they mould
Rock n roll is our epiphany
Culture, alienation, boredom and despair