On a glowering November morning — sky bruising, wind whipping, rain threatening — Coldplay’s Will Champion shows up at Sydney’s Coogee Beach as a perfect counterpoint to the weather. The drummer is sunny and solicitous under his trucker hat, strolling along the famous path toward Bondi with a gentleness and courtesy (“Oh no, after you!” “Do you need a hand?”) one might not entirely expect of a man who hits things for a living. The night before, toward the end of Coldplay’s fourth Music of the Spheres show at Sydney’s Accor Stadium,...