Michael Keaton's not-so-superheroAwards season officially begins today with the opening of the 71st Venice Film Festival, which in recent years has put the likes of Black Swan and Gravity on the launching pad with its prestigious first-night slot. Alejandro González Iñárritu’s latest, Birdman (Or The Unexpected Virtue Of Ignorance) perches beautifully in that company, being an experimental yet fully grounded work by a supreme visual stylist.Like those two films, it also puts its faith in a bravura performance, this one from Michael Keaton, who literally strips himself to the essentials in a role that stamps this as the first must-see of the autumn.The trailer suggests a very different film from the one Iñárritu actually made, highlighting a magic realism and sense of mental turmoil that does feature in the film, but which doesn’t dominate it.