Timberlake was trained as a child as a performer, mostly by Disney. Almost anyone who was on the Mickey Mouse Club, which Timberlake was, is going to have familiarity with the sketch comedy format - and learn that they need to be a pro about work if they want to stay in the industry. Bieber is a pretty kid with a pretty voice from a small city who was lucky enough to be spotted on YouTube. It’s obvious that, aside from whatever he does in the studio, he’s getting no guidance at all. Spaghetti Monster help him when the gravy train stops.