Karen Holmes Ward, host of WCVB’s “CityLine,” introduced a special screening of “12 Years a Slave” at AMC Boston Common on Wednesday night. The film is based on the memoir by Solomon Northup about a free black man from upstate New York who is abducted and sold into slavery. Cast members include Chiwetel Ejiofor (as Northup), Michael Fassbender, and Brad Pitt. Museum of African American History Executive Director Beverly Morgan-Welch led a discussion after the early look at the film, which has its Boston release Friday.Mark Shanahan can be reached at email@example.com. Follow him on Twitter @MarkAShanahan. Meredith Goldstein can be reached at firstname.lastname@example.org. Follow her on Twitter @MeredithGoldste.