Purefoy will be playing Kantos Kan, the captain of the Xavarian, the grand flagship of Helium. Carter meets him in the prisons of Warhoon, and they're forced to battle in a gladitorial match. They become allies as only former gladiators can, and Kan assists him on his quest to save Mars / Barsoom. I found it difficult to picture the secondary characters in Princess of Mars, so I feel lame in saying "Purefoy is perfect as Kan!" but he'll fit the part of a seasoned naval captain. He'll be quite dashing, even. (Actually, I think Purefoy would have made a good Carter.)
Church will be playing Tal Hajus, who THR calls "a vicious Thark warrior who is determined to become king." In the books, he's a Jabba the Hut type who lusts after Princess Dejah Thoris, and is happily ensconced as a leader, so this might be one of the first big changes you'll see in the story. It would probably make things a lot more interesting to have Church play him as a capable bastard than a slobby wreck.
Read more over at SciFi Squad