Marcus Smart doesn’t take his eyes off the court. Not once. Seated under the basket, he’s polite—warm, even—in answering questions on this February morning. But a faint scowl lines his face. Not in a rude way. In a how can you stare at a court and not want to tear it up? kind of way. It’s 10 minutes before shootaround. Nine hours until the Celtics play the Cavaliers in Cleveland. But when Smart is near a court, he loses track of time. Loses himself between the lines.Enough talking. He’d fly off his chair right now and chase down a loose ball if he could. Screw his knees. His ankles. His arms. His back. Any ligament or joint in his body, really. If a ball is rolling, Smart is diving.