EXCLUSIVE: Sam Mendes has picked Johnny Flynn (Emma, Beast), Tuppence Middleton (Downton Abbey, Mank) and Sherlock’s Mark Gatiss to portray legendary stars Richard Burton, Elizabeth Taylor and John Gielgud in The Motive and the Cue, a mouth-watering new play by Jack Thorne that explores how acting giants Burton and Gielgud staged Hamlet on Broadway in