Continuity mistake: When the girls have the trucker follow them, they are shown sitting on top of the car. However, when the driver is walking up to them, they are shown getting on top of the car again. (01:50:22 - 01:50:51)
Audio problem: When Thelma approaches the Highway Patrolman's window you can hear her cocking the revolver she then points at his head, but the revolver remains visibly uncocked.
Chosen answer: It's just as you said; he was telling the others to hold their fire. He had no idea that they were going to drive off the cliff; his reaction was evidence of this.
zendaddy621