While Sean Gunn isn’t doing straight motion capture, his acting and facial expressions are used by the digital effects artists when it comes to creating Rocket in post-production.
While James Gunn’s post is absolutely meant to poke some fun at his brother, that’s what brothers do, that’s not the only reason he’s included the clip.
The director also wants to correct a misconception that Sean Gunn’s only purpose is to act as a stand in, so actors have something to work against.