You have seen Chatham in films such as The Help and W.
Henson looks more familiar as he has been seen back as far as the Mighty Ducks trilogy. Since then you've seen him in Idle Hands, The Butterfly Effect, She's All That, and most recently in Jobs. He has been formerly billed as Elden Ratliff and Elden Ryan Ratliff.
Now to the important stuff... Castor and Pollux aren't huge characters, but they do serve their purpose. The two are brothers and work closely with Cressida (Natalie Dormer) and Messalla (Evan Ross). They are both Capitol cameramen who join the rebellion and film propaganda pieces by following Katniss. Pollux and Castor are each a former Avox*.
*If you don't know what an Avox is, they are basically runaways from the Capitol and have been punished for rebelling. They have their tongues cut out and are Capitol servants who serve the tributes and Capitol citizens.
