It’s a different battlefield from what they’ve known before, and a different kind of movie set too. Perched atop a series of trampolines, with bouncy surfaces beneath their feet, and red rose dodgeballs in their hands, Robert De Niro and Christopher Walken are standing side by side on screen for the first time since 1978’s The Deer Hunter. In that earlier movie, they played young men who eagerly went off to war, but now they’re in a different kind of conflict here. They’re in the midst of The War with Grandpa’s biggest battle.