Surprising Opening

If your opponent's choice of opening is not the one you expected, then a lot depends on the flexibility of your opening repertoire. If you have a very narrow repertoire (i.e. only one line against everything) then generally it's best to just stick to what you know. It's true your opponent might have something specific lined up against this, but you can rely upon your greater general understanding of the positions - something that will increase in importance once any preparation from your opponent runs out. If you do have a wider repertoire it might be a good practical choice to use this. This could negate all or most of your opponent's preparation, and still lead to positions in which you generally have more
experience.