By SI Staff Five questions on Celtics-Warriors heading into a pivotal Game 5. The NBA Finals shift back to San Francisco for Game 5 tonight, with the Celtics and Warriors tied 2-2. What to make of the series so far? Which role players need to step up? Who will win? Our writers weigh in. What’s surprised you most in the Finals?Howard Beck: The relatively muted performance of Jayson Tatum. His stats across the board are solid—22.3 points, 7.0 rebounds, 7.8 assists—but it just doesn’t feel like he’s made an imprint on this series. He’s the Celtics’ best player. He finished sixth in the MVP voting…