All of the close-up shots of Sarah's hand holding her phone were shot on one afternoon, in the library, and yes, it was actually Anna Kendrick's hand.
The paint ball hits on Billy (Paul Wesley's) neck and chest were simulated by cutting from a blank fire (air only) to a shot of paint blown from a drinking straw hitting him.
When Sarah comes into the hobby shop alone to speak with Billy, one of Billy's friends is missing. He was supposed to be in the scene, but missed his train from Chicago that day.
Paul Wesley is billed second, although he's a supporting character and not the male lead, and has quite limited screen time.
Chuck Carter used a real, loaded, paint ball gun to shoot Paul Wesley in the leg, but the pressure was dialed down, and Paul Wesley had a fiberglass protective plate covering his leg. After shooting the scene, Chuck Carter said he had been very tempted to "accidentally" aim high and hit the unprotected part of his leg!