Answer by taking a quote from the following article:

Good Night, and Good Luck is a 2005 American historical drama film directed by George Clooney and starring David Strathairn, George Clooney, Robert Downey, Jr., Patricia Clarkson and Jeff Daniels. The movie was written by Clooney and Grant Heslov (both of whom also have acting roles in the film) and portrays the conflict between veteran radio and television journalist Edward R. Murrow and U.S. Senator Joseph McCarthy of Wisconsin, especially relating to the anti-Communist Senator's actions with the Senate Permanent Subcommittee on Investigations. Although released in black and white, Good Night, and Good Luck was filmed on color film stock but on a greyscale set, and was color corrected to black and white during post-production. It focuses on the theme of media responsibility, and also addresses what occurs when the media offer a voice of dissent from government policy.

In September 2005, Clooney explained his interest in the story to an audience at the New York Film Festival: "I thought it was a good time to raise the idea of using fear to stifle political debate." Having majored in journalism in college, Clooney was well-versed in the subject matter. His father, Nick Clooney, was a television journalist for many years, appearing as an anchorman in Cincinnati, Ohio, Salt Lake City, Utah, Los Angeles, California, and Buffalo, New York. The elder Clooney also ran for Congress in 2004.  George Clooney was paid $1 each for writing, directing, and acting in Good Night, and Good Luck., which cost $7.5 million to make. Due to an injury he received on the set of Syriana a few months earlier, Clooney could not pass the tests to be insured. He then mortgaged his own house in Los Angeles in order to make the film. Dallas Mavericks owner Mark Cuban and former eBay president Jeff Skoll invested money in the project as executive producers. The film ultimately grossed more than $54 million worldwide.  The CBS offices and studios seen in the movie were all sets on a soundstage. To accomplish a pair of scenes showing characters going up an elevator, different "floors" of the building were laid out on the same level. The "elevator" was actually built on a large turntable at the intersection of the two floor sets, and rotated once the doors were closed. When the doors reopened, the actors appeared to be in a different location. In doing so, the movie exercised a bit of dramatic license--the CBS executive offices at the time were located at 485 Madison Avenue. CBS News was located in an office building just north of Grand Central Terminal (demolished and now the site of the Met Life Building); and the See It Now studio was located in Grand Central Terminal itself, above the waiting room. For dramatic effect, all three areas were depicted as being in the same building.  Clooney and producer Grant Heslov decided to use only archival footage of Joseph McCarthy in his depiction. As all of that footage was black-and-white, that determined the color scheme of the film. A young Robert Kennedy is also shown in the movie during McCarthy's hearing sessions. He was then a staff member on the Senate subcommittee chaired by McCarthy.

Who plays the role of Robert Kennedy?
He was then a staff member on the Senate subcommittee chaired by McCarthy.