Do professional level chess players mainly stay consistent in their opening play? I know that professional chess players obviously have to have a wide repertoire and try to know as much as they can, but I was curious if they mainly stay consistent.  The reason for asking this was because I was watching the following video on Bobby Fischer and in game 6 of the world championship against Spassky, everyone was shocked when Fischer played the English Opening. It even shocked Spassky as he was not prepared to handle the English and according to the video gave Fischer a standing ovation for outstanding play, so do players of professional level normally stay consistent?  Has this changed from the past until now?
No two people are alike, and no two chess players are alike either.

To begin with, world class grandmasters study chess all the time, and they can play just about every opening under the sun and score incredibly well.  

However, in order to get the very best results possible, most grandmasters will study and memorize moves and plans for a handful of openings for each color and each 1. e4 and 1. d4.  By doing this, they gain a slight advantage over their opponent in terms of a better position, using less time, or sometimes just a psychological edge.  

Since the top players spend so much time learning these openings, they tend to not switch openings all that frequently.  Changing one's repertoire would require redoing all the studying and memorization that had already been done.  Frequently though, as other players discover and show new ideas, the repertoires evolve.  A player might substitute one line for another or find an improvement in some opening line.  The advent of super-strong computers has definitely increased the speed with which opening lines change.  Nowadays players must constantly be looking for improvements and tweaking their lines in order to stay on top of things.  Another factor is the internet - minutes after completion, the game is available for anyone in the world to see.  Surprises can generally only be used once.

To discuss Fischer specifically, his entire career he mostly played 1. e4 and the King's Indian Defense and the Najdorf Sicilian as black.  His openings did evolve and change over time, but he was fairly consistent, especially with 1. e4.  For this reason, when he played 1. c4 against Spassky in the '72 World Championship, it was a huge surprise.  Spassky had spent his time figuring out how to meet 1. e4 and had to readjust to play the Queen's Gambit Declined.