This strategy would certainly muddle the process of shooting, as characters both human or otherwise may be absent during scenes, and then added during the editing process.
With Avengers: Infinity War uniting all the separate Marvel properties, it’s a logistical nightmare to get all of the respective A-listers together for large ensemble scenes.
So perhaps Sean Gunn is referencing the fact that actors will be added to scenes in post through CGI.
Honestly, how are the Russo Brothers expected to actually assemble every member of the Avengers, Guardians, as well as Thanos’ side of things?