I was slightly disappointed with the initial casting of John Carter of Mars, but as Andrew Stanton keeps bringing the wonderful with every addition to the roster, I just have to trust him when it comes to his choice of Taylor Kitsch. But Stanton is sending more impressive talent to Mars, as according to The Hollywood Reporter, James Purefoy, Thomas Haden Church, and Mark Strong have joined the cast.
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.
Last but not least, Strong will be playing Matai Shang, the revered ruler of the Thems. I sped through the series and I don't think he appeared until later, but Stanton was open about picking out characters to develop for sequels, so he'll undoubtedly be one of them.
Purefoy, Church, and Strong round out the cast that includes Kitsch as Carter, Lynn Collins as Dejah Thoris, Willem Dafoe as Tars Tarkas, Samantha Morton as Sola, Dominic West as Sab Than, and Polly Walker as Sarkoja. It's great casting all around (Walker will be absolutely terrifying as Sarkoja), and I can't wait to see filming begin.