The Doctor mentions The Master and his crimes to Amy, who immediately knows what he's talking about. Although Amy has never met The Master, The Doctor could have discussed the matter off-camera.
When The Doctor and the Gunslinger are facing each other down at high-noon, the clock strikes 12, but all shadows and the brief glimpse of the sun do not indicated it actually being 12 P.M.
The Doctor grabs a gun from Walter's holster, and we hear him cock it. In the next shot, it isn't cocked and we see him cock it. In the next shot - again - it isn't cocked, and we see him cock it a third time.