The film rolled, the mistakes played out again on the screen, but the solutions didn’t just magically appear when the lights went up. This isn’t the movies, after all. It’s basketball, and the Celtics still aren’t sure exactly why they have collapsed in the second half of their two first-round playoff losses against the New York Knicks. They don’t have much time to figure out an answer, either. The clock is ticking, and the Celtics are staring at an 0-2 hole in this best-of-seven series. After Tuesday’s 87-71 loss, they returned to Boston, where they will play Game 3 Friday at TD Garden, and reviewed film. Then Doc Rivers spoke with reporters on a conference call, offering up as few answers as he did the night before. “Listen, we have to do something,” the coach said in reference to the two second halves, in which his team has scored a combined 48 points — an average of 12 per quarter. “We have to do something because we’ve come out flat, period. We’ve done all the other stuff, but we’ve come out flat [after] back-to-back halftimes. “As a coach, that’s an alarm and I don’t know what it is. But we have two days to figure out what it was.” Perhaps Rivers should ready a rousing halftime speech to rally the troops. “If it’s going to come to that, we’re in trouble,” he said, laughing. The Celtics shot a combined 14 for 62 (22.6 percent) after halftime in the two games. And according to the Elias Sports Bureau, the Celtics are the first team in the shot-clock era to score 25 or fewer second-half points in consecutive games either during the regular season or the postseason. Also, the Celtics’ 23 second-half points in Game 2 set a dubious team mark; it was the lowest output in franchise history in the postseason. “The game, it’s not as difficult as we make it at times,” Rivers said.