Firstly, a synopsis and list of characters:
Plot Summary: Michael Clayton, an attorney at a high-powered law firm, known as "the janitor" because he is the one that is called in to fix messes that no one else can handle. However, when Michael's friend and old associate, Arthur Edens, goes off his meds, melts down in a deposition, and starts helping the plaintiffs build their class action case against Agtek, a chemical company that is the firm's most lucrative client, Michael has his work cut out for him. Arthur's sudden "suicide" has Michael wondering what role Agtek in general and Agtek executive Karen Hauer have played in Arthur's death.
Henry Clayton (Michael's young son), Mr. Verne (head of a covert security firm), Arthur Edens (Michael's friend and old associate), Barry Grissom (a senior partner at law firm), Mr. Iker (works with Mr. Verne), Gene Clayton (Michael's younger brother), Gabe Zabe (loanshark), Brini Glass (young attorney and Michael's lover), Anna Kysursen (she is part of a class action law suit against Agtek), Mr. Greer (client at Michael's law firm), Jerry Dante (a local lawyer), Don Jeffries (Corporate Titan at Agtek), Detective Montes (a friendly cop), Timmy Clayton (Michael's cousin), Adrise (Michael's assitant), Jeff Gaffney (partner at Michael's law firm), Raymond Clayton (Michael's father), Stephanie (Michael's sister), Ivy (Michael's ex-wife), Gerald (Now Ivy's husband).
I'm guessing that Ms Ehle plays Brini Glass. In an article with more depth about the characters, Brini Glass is described like so. Warning, that link contains mega spoilers. [edit: oops, linked to the wrong place, fixed now]
Brini Glass: Late 20s-early 30s. An attractive young attorney, she's a gogetter who is having a secret affair with her associate, Michael. Not really clear about Michael's role with the firm, Brini pressures him for some information and gets a little more than she'd bargained for.
Note that unlike some other characters, Brini is not described as a "lead".
The film is inspired by a real court case according to Mr Clooney:
"It's actually based on a real court case — not directly based on it, but based on the idea," Clooney said of the film's origins. "There was a very specific court case, that we won't name to save the lawsuits, from a very famous prosecutor who at the time was a defense attorney. [A character] basically has information that he knows will destroy his client and hides it. I play one of the lawyers who works for that law firm, who gets that information and realizes I'm defending a group of people who are truly guilty." Clooney, currently starring in the journalism drama "Good Night, and Good Luck" (which he directed and co-wrote), added that pushing such buttons always gets him excited about a project. "Those are always the fun ones," he laughed, "when you realize you're in trouble."
Actually the Clooney Network (obviously more on the ball than us!) has a whole section with archived articles about the film. From it, some interesting information I've gleaned is that it's made by the same team as Syriana and Traffic and the director also did the Bourne movies. The film is produced by Section Eight, Samuels Media, Mirage Enterprises and Castle Rock Entertainment. Summit is the principal distributor and has sold rights to Medusa, Pathe, DeAPlaneta and SND/MG in Italy, the UK, Spain and France respectively. In Germany Constantin has film rights and A Company has TV rights. Ren TV has TV rights in Russia.