Helmer Steve McQueen continues to fill out the ensemble of his drama “Twelve Years a Slave,” as Paul Giamatti and Sarah Paulson have joined the cast of the New Regency pic based on Solomon Northrup’s 1853 nonfiction tome.Duo joins Chiwetel Ejiofor, Michael Fassbender, Brad Pitt, Benedict Cumberbatch, Scoot McNairy, Ruth Negga and Garret Dillahunt. Ejiofor stars as the book’s author, a free man kidnapped and sold into slavery. Paulson will play Fassbender’s jealous wife, while Giamatti will play Freeman, who takes possession of the slaves upon their arrival in New Orleans. McQueen co-wrote the script with John Ridley (“Red Tails”). Dede Gardner and Jeremy Kleiner will produce for Plan B and Bill Pohlad will produce for River Road. Paulson, who appeared in the HBO telepic “Game Change” and Sean Durkin’s “Martha Marcy May Marlene,” was recently named a series regular on the upcoming season of FX’s “American Horror Story.” She’s also attached to star alongside Frank Langella in FilmColony’s “The Time Being.” Giamatti, who recently wrapped Amber Entertainment’s “Romeo and Juliet,” will next be seen in WB’s “Rock of Ages” and Greenstreet Films’ “Lucky Dog.” UTA and Kipperman Management rep both Paulson and Giamatti, who is also repped by attorney Robert Offer.