Full face makeup can fully transform an average human into an alien creature, monster or robot, but this transformation comes at a price- and it's the dialog that pays in the end.
Mumbled, butchered and muttered incoherently, these lines of dialog lose all weight when the actor saying them can't even move their lips under all that makeup.
And yet it's still better than those wacky overdubs they used to do back in the 60s and 70s! (NSFW due to language)