John Wesley on Genesis 15:12

And when the sun was going down - About the time of the evening oblation. Early in the morning, while the stars were yet to be seen, God had given him orders concerning the sacrifices, Ge 15:5, and we may suppose it was at least his morning's work to prepare them, and set them in order; which when he had done, he abode by them praying and waiting 'till towards evening. A deep sleep fell upon Abram - Not a common sleep through weariness or carelessness, but a divine extasy, that being wholly taken off from things sensible, he might be wholly taken up with the contemplation of things spiritual. The doors of the body were locked up, that the soul might be private and retired, and might act the more freely. And lo, a horror of great darkness fell upon him - This was designed to strike an awe upon the spirit of Abram, and to possess him with a holy reverence. Holy fear prepares the soul for holy joy; God humbles first, and then lifts up.