When I first read that Black Crusade would contain rules for playing CSM and human cultists, I had assumed that all of the PCs would play one or the other, and that there would be a significant gulf between power levels, with human characters around Dark Heresy/Rogue Trader level, and the CSM around Deathwatch power level. With the release of new information, it is now apparent that the intention behind BC is to play a mixed group of CSM and humans, which presents a number of issues which are either obstacles or roleplaying gold depending on your perspective.
CSM and humans working together - any chance of equality at all?
In all of the fiction I have read set in the 40k universe, CSM usually treat humans as either minions and slaves at best, or target practice and food at worst. They generally push them around, kill them with impunity, and treat them as an inferior, lesser species. This is true even of regular SM, although to a lesser degree. I imagine a mortal worshipper of chaos would have to achieve an incredible level of power to impress a CSM, and even then, his mortal nature would render any respect payed to him fairly precarious, because no matter how accomplished a human is, they willl never be accepted into the ranks of the Astartes as equals, because they do not possess the training, the gene-seed and the genetically and ideologically-enforced sense of brotherhood. They are simply not part of the clique, and never can be.
So what kind of status is a mortal PC going to have in a party filled with immortal superhumans who generally believe in a philosophy of might makes right? Is there going to be equity and discussion amongst the PCS, or are the CSM simply going to tell the humans how it is? Even in a RPG, where it is expected that the PCS make decisions on a generally democratic basis, how is is possible to roleplay a centuries-old superhuman who respects the opinion of someone who in their worldview is likely a fragile and infantile being, whom they could dismember in a heartbeat? How could the human PCs not live an existence of constant terror and alienation in the presence of these inhuman, immortal killing machines?
Of course, this could all make for fun roleplaying. Still, our group plays CSM using Deathwatch rules, and no-one is putting their hands up to play a mortal cultist on a permanent basis (considering how the PCS treat the mortal members of the crew, this is unsurprising).
I'd like to hear other people's thoughts on how they have, or would, approach a mixed CSM/human party.