Bringing together a host of characters from different films, and doing it coherently, is not easy, but Joss Whedon managed it beautifully in Avengers Assemble. Now, three years later, the director is back with Avengers: Age of Ultron, where Captain America/Steve Rogers, Iron Man/Tony Stark, Thor, Black Widow/Natasha Romanov, Hulk/Bruce Banner and Hawkeye/Clint Barton battle…