J.R. Smith, who is 6-foot-6 and muscle-armed, who can dunk a basketball out of a standing jump and, when necessary, can outrun nearly everyone on the court, is an oddly unimposing man. He is slender and narrow-eyed. He carries himself with sloping shoulders, unhurried. If his deadpan look is a pretense, a tactic meant to entrap the opponent, then the Knicks — filled with hope and bravado as they start their playoff season today — have discovered its virtues, finally. This is no small revelation for a team that has struggled since the generation of Patrick Ewing and the erratic John Starks to find a consistent option to its star player. At Madison Square Garden this afternoon, the Knicks and Celtics revive a rivalry. The Celtics will lead with Kevin Garnett and Paul Pierce, and the Knicks, of course, with Carmelo Anthony, the NBA’s leading scorer. Smith will be waiting on the bench when the game starts, and once installed in the lineup he will be the one lurking on the wing. Once considered by conventional wisdom to be too selfish or too cocksure, or even disobedient, he has become Anthony’s surefire sideman. Smith, 27, was the team’s second-leading scorer with 18.1 points a game. Alongside brittle, older teammates, no Knick played more minutes than Smith this season. And in the past five weeks, when he averaged more than 24 points, he has done something rare for a professional athlete — transforming his game in the middle of the season. "Yes, it feels good," he said yesterday after the Knicks’ last practice before the postseason. His voice was muted. "Yes, because I’ve waited so long to hear good comments about me."