Given that Sarsgaard and Hammond may not exactly be household names for all film-goers, here's a little background on them both. Sarsgaard has gained plaudits for his roles in Boys Don't Cry, Shattered Glass, and Jarhead. He also played Zach Braff's best friend in Garden State and is married to Maggie Gyllenhaal.
Supervillain Hector Hammond has perhaps one of the most lamentable superpowers in the DC Universe. A strange meteorite gave him the whole gamut of psychic powers (mind control, telekinesis, etc.), but it also afflicted him with an epic case of macrocephaly. In fact, Hammond's head is so heavy that he must sit strapped to a chair at all times - otherwise, his domepiece will crush his body. In the comics, Hammond was a lowly criminal prior to gaining superpowers. His film counterpart is a pathologist who's considered a disappointment by his senator father.
Sarsgaard is the most recent addition to director Martin Campbell's 2011 GL flick. Last week, Blake Lively joined the cast as Carol Ferris a.k.a. Hal Jordan's romantic interest a.k.a. the on-again, off-again Green Lantern antagonist Star Sapphire. It's unconfirmed whether Hammond or Ferris will appear in their civilian or villainous incarnations.