It goes by and large without saying, but when you're a kid watching Doctor Who, the news that the Doctor is going to actually die at the end of the episode is the most exciting thing ever. Obviously this is down to two big reasons- 1: Most other shows don't have the luxury of killing off the main character and not... you know... ending. The sheer novelty value alone of discovering that the title character of your favorite show could very well be brutally killed off has rocked many a 12 year old's world. 2: The old Doctor being killed off means that there's a New Doctor coming, and we don't know anything about him! If you're the sort of 12 year old boy that has just spent the last six months making lists of stories with Cybermen in them and cross referencing that to stories written by Robert Holmes before outputting that data in a bar chart* then the sheer amazing thrill of NOT knowing what the Doctor was going to be like from this moment on was pretty much like crack.
* Not that I would ever have done this... However, while the changeover from one Doctor to the next was a rare and revered thing, and like the best china only to be brought out once in a blue moon, the companions were a different matter. Add to that that in those blissful pre-internet days, you were actually far less likely to know in advance whether that week's particular episode meant that a companions time was up. My point is that the departure of a companion was - and is - a big deal. (Or at least it has been since Innes Lloyd left the program.) As a viewer you tend to have a lot invested in them - it's only natural that their leaving carries some significance. That said, for something that is that significant, various production teams have managed to thoroughly screw the pooch over the deal on more than a couple occasions. Here then is a quick reference to the things that don't work. We know they don't work because we've watched them ruin companion departures before. Please stop making these mistakes now. Some terribly old spoilers within.