It's a frustrated director's fantasy come true: a light conveniently falls on a cast member's head during a play's rehearsal, putting an end to some excruciating line readings. Putting to a merciful end, if only temporarily, the bad actor. Suddenly, a chance to cast someone good in the hack's stead. But whom? Woody Harrelson? No, Woody has already committed to the next Hunger Games sequel. Michael Fassbender? Nope - X-Men. Jeremy Renner, perhaps? Sorry - he's busy with his Avengers pals. So many super heroes, so few brave actors to be found. This is the dilemma, in more ways than one, for Riggan Thomson, adaptor, director and star of the play within the film which is Birdman, or (The Unexpected Virtue of Ignorance) . Thomson himself knows something about superhero movie franchises. The middle-aged actor played the fictional character Birdman in three films during more the more youthful and prosperous years of his career. But he did turn down