So the World Championship Match comes down to game 12, although playoffs are a distinct possibility. And it is the existence of playoff games that have certainly changed how final World Championship games are now played.
In an earlier time, a tied match meant that the Champion retained the title. Therefore, if the match reached game 24, it was a do or die game for at least one player. In the case of the Karpov v Kasparov matches, Kasparov won the title in 1985 by winning game 24 (and the match 13-11), and retained the title in 1987 by repeating the feat, but in this case drawing the match 12-12.
In both cases there was a sense of anticipation about what strategies both players would employ, but in both games, Kasparov did not deviate from his normal style, a wise decision as it turned out.
I suspect this will also be the case for game 12 of Carlsen v Karjakin, as a switch to rapidplay games may be agreeable to both sides. Probably 1.e4 will be played although there is also scope for a real surprise. Back in 1987 I can remember see "Guess the Opening for Game 24" competition at the Melbourne Chess Club, and while most choices were pretty main stream, one enterprising soul plumped for 1.g4!