I’m not sure of all the timing involved in what gets chosen when in each and every case. I’m just going by what I have heard various folks discuss about the process.
I know that from the time they do the voice work until the finished product is in tjeaters is several years-much longer than regular movies.
And I am pretty certain that the voice characters are chosen before pretty much anything else because some of the actors spoke about how all they had to go on were some general rough sketches in storyboard form along with the script for some of the more complex action sequences.
I’m thinking particularly of the sequence where they are being chased by the dragon while trying to escape the castle.
Also, many times actors in the same film don’t necessarily physically work together in the recording process either.
This is one of the reason why a lot of actors love being voices in animated films. No getting dressed up, no elaborate rehearsals or blocking shots etc. which can make regular film work grueling.
The other reason they gave is that they look really cool in the eyes of their young children.
:D