Aaron Eckhart will join Sigourney Weaver in “The Mercy Seat.”The Neil LaBute play opens MCC’s season on Dec. 18 at the Acorn Theater, a new Off Broadway venue on West 42nd Street. Eckhart will play a married man who begins his affair with an older woman on the day after the terrorist attack on the World Trade Center. “The Mercy Seat” replaces LaBute’s “The Distance From Here” on the MCC sked; performances run Nov. 26 through Jan. 11. The new Off Broadway season is shaping up to be more movie-star laden than the one on Broadway. Al Pacino, Chazz Palminteri, Billy Crudup and Steve Buscemi headline the National Actors Theater’s production of “The Resistible Rise of Arturo Ui” this October at Pace University.