Shohreh didn't have a single lead role in a movie or TV show beforehand. She wasn't even B list at the time of Expanse despite a long successful career. Karl Urban is B. He had Dread 15 years earlier where his face was covered. Trek was supporting. Thor was a minor role.

The A, B, unknown tiers have nothing to do with the actors ability but their current popularity to get viewers to watch what they are in.