On ABC this evening, the opening hour is overrun with thespians, as characters on both sitcoms conspire to put on plays. First up, on The Middle, is Sue Heck (Eden Sher), who teams with Brad (Brock Ciarlelli) to not only co-direct and co-write, but co-star in, a senior high school play they’re calling “The Loneliest Locker.” The question is, will it be a dramatic triumph – or a combination destined for embarrassment?