Linus Roache (“Batman Begins”) has joined the cast of “Law & Order.”Thesp will play an exec assistant D.A., filling the role left by the promotion of Sam Waterston’s Jack McCoy character. McCoy is being upped to D.A. following the departure of Fred Thompson. “I’ve known Linus’ work for several years,” said “Law” supremo Dick Wolf. “He is an actor who totally gets inside his roles. I think he and Sam are going to raise the bar and add intellect and passion in the back half of the show.” “L&O” is slated to return for its 18th season early next year. Roache had been set to appear on Fox’s midseason drama “Canterbury’s Law.” That role will now be reduced to a recurring or guest gig.