They just use extras; they'll usually let it be known somewhat publicly that they will be shooting in that area for a day or two. Then they have everyone agree to a basic contract before just shooting on that street, using pedestrians as extras. Although, I imagine period pieces would have more extraneous casting calls.
"Somebody told me when the bomb hits, everybody in a two mile radius will be instantly sublimated, but if you lay face down on the ground for some time, avoiding the residual ripples of heat, you might survive, permanently fucked up and twisted like you're always underwater refracted. But if you do go gas, there's nothing you can do if the air that was once you is mingled and mashed with the kicked up molecules of the enemy's former body. Big-kid-tested, motherf--ker approved."
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