Apparently this was a rehearsal for the scene, so the extinguisher was a prop one (hence the moment Miranda sassy looks at the camera/crew for how he was acting her in panic), so they went on to it.
But someone had switched the extinguisher to the actual fire thrower that would be used on the scene (I dont remember if was proposital or not or simply a mistake because they were meant to be identical).
So this is their actual reaction, cause when he pressed the valve, he really wasnt expecting to fire come out, neither was Miranda, so their reaction is 100% genuine and they did freak out as on tape.
The thing went out so well they were unable to reenact their reactions or surprise as the first time on rehearsal, so the rehearsal scene was kept and put it into the actual episode.
