Dorothea Graves: played by Archie Panjabi, Dorothea is a recently widowed half-Indian, half-white-British anthropologist on the hunt for her husband's murderer.
Patrick Graves: played by Simon Woods, Patrick was Dorothea's husband, lately murdered, and a rising political star on his way to Downing Street.
Winslow Spencer: played by Ioan Gruffudd, Winslow--or Winnie, as Dorothea calls him--is Patrick's closest friend, who unfortunately has a taste for interesting business dealings.
Rai Nazir: played by Shahrukh Khan, Rai is the head of a shadowy group called the Thanivali who may or may not have had a hand in Patrick's death.
Charlotte Alexander: played by Samantha Mathis, Charlotte is an established politician and was Patrick's running mate. The media and public at large suspect the two of having carried on an affair.
And that's how Diana casts it. All images pulled from Google and Wikipedia.