The mail shipment contained video footage of a player Terrence Williams was told to study. And that player was Paul Pierce. Williams was a high school senior in Seattle — a hike up Interstate 5 from where the Celtics will be playing the Portland Trail Blazers on Sunday — when those tapes arrived. Louisville coach Rick Pitino, whom Williams had committed to play for, sent the film, and from it, Williams said, he learned how to play on the elbow (near the free throw line) where Pierce has made his living for years. He also learned how to come off screens and learned footwork. When Williams reached the NBA, he told Pierce each time they played that he had watched film of the Celtics forward. Now, after Williams formally signed a 10-day contract with the Celtics Wednesday, the two are teammates, at least for now. But the way the 25-year-old has played in two games thus far for the Celtics — averaging 5.5 points, 3 rebounds, and 3 assists in 18.5 minutes — he is making a strong case to stay a Celtic beyond March 1 when his contract is up.