No need to repeat daniel-p's comments, above, but he well summarizes the process ... or at least the way it's supposed to work according to WTS instructions.
It's doesn't always, of course, because of the difference in congregational personalities and human resources.
Our old congo went through three distinct personalities in a decade:
1. We had an young, energetic elder who had been at Bethel before he got married and had kids, so whenever a speaker did not show up (it was a rural area and it snowed in winter, so this happened frequently during the cold weather months) we started the WT study while he went into the library and "whipped something up." He told me once that the easiest way to do this was to pull a bound volume of the WT down from the shelf and skim through the study articles. It was his theory that the study articles were basically manuscript talks and that by highlighting the key points and using the highlighted material as an outline, one could produce a public talk in no time. It helped that he was bright, glib and personable and a hell of a speaker.
2. After he and some other servants left for greener pastures, we were left with only three elders all of whom were under 40 and none of whom was much of a speaker. So for the next couple of years, whenever there was a PT no-show, we held the WT study and called it a day. And y'all are right; Everyone loved this! It was like a snow day at school when we were kids!
3. Some years went by, the congo grew by leaps and bounds (this was back in the 70's and early 80's when there was actual growth) and our elder body swelleld to 10 or 12, including a couple of ex-Bethelites, one of whom had been a missionary. These guys were really organized and handled the PT schedule in the exemplary fashion outlined by the WTS. They only brought in the very best speakers, and they called them in advance to remind them. As a result we seldom had no-shows, and if the weather was bad they assigned somebody in advance to be prepared to go on, just in case. Looking back, our public talks in those days were almost always really good. These two guys were prominent in the circuit affairs and pretty much ran the circuit and district assemblies in our area, and they were the gatekeepers to who got to appear on the assembly and convention programs, so they when they called other congos to get speakers they got VIP treatment.