That actually reminds me of a… I think it was a comic, but it might've been a prose story. It's set on an agri-world. The people know the stories of the Emperor's Angels, the Space Marines. One day, the planet is attacked by an Ork Waaagh! and the Black Templars are dispatched to deal with it. The story glosses over the majority of the battle itself, instead focusing on the reaction of the people to the Marines. They're terrified. These giants in armor just mowing down monsters that had demolished the PDF. The story closes with the people praying to the Emperor in thanks for their salvation, but also that they would never have to see the Angels of Death again.
See, this is what Marines look like to people on the outside.
But for a Marine roleplaying game (and Marine focused novels etc.) most of the time we are going to be on the inside.
Er, slike how to most of the Imperium Inquisitors are legendary figures who swoop in from nowhere to dispense terror/enact the Emperors will.
But those on the inside know that there is a constant war being fought between Inquisitors over what is best for the Imperium. There are factions and politics and secret battles that the average citizen doesn't see. That Inquisitors themselves have distinct personalities and ways of doing things.
So, we would want the same for Marines. Even if they look like impassive Angels of Death to the casual observer, amongst themselves and their peers they have a lot to talk about and want and politic about and disagree to the point of shooting each other over.
(Generally, I think it is safer to er on this side of personality. If you write someone as looking like a personality free killing machine, people seem to assume that is what they are. Where as if they were fighting their battles on twitter, there could be no argument that they think for themselves.)