Specifically, if Notary is a member of the Black Order but also Thanos’ right hand man, he could be doing a motion-capture performance as Corvus Glaive.
Glaive is basically immortal in the comics, posing another great threat to the heroes, if this does turn out to be Notary’s role.
However, this is merely speculation at this point, but with Notary confirmed to be involved (combined with the other information), this does seem like a strong possibility.
IMDb is not the most reliable source for casting information as anyone can add/change credits, but it does make the situation that much more interesting.