Continuity mistake: When the armor truck driving thief starts to come to on the hood, he lifts his head straight up. But in the next shot his head is back down and tilted and he lifts it up again.
Continuity mistake: After the end credits roll there is a brief scene similar to the opening, with a building being blown up. Riggs and Murtaugh back out of the area just as the explosion starts and completely leave. There is a high angle shot and a police car appears where they just were. It wasn't there before.
Answer: Billy was played by Mark Pellegrino.
Bishop73