A question

I keep thinking about the start of my games, and I keep coming to a question that I thought I'd ask anyone who comes across it.

In a game, white gets the first move, and he uses it to begin his opening, wether it's a scotch or sicilian or whatever. But if black then makes a move that doesn't conform to white move, does white continue with the opening, or does he react to blacks move?

Like,

1. e4 e52. Nf3 Nc63. d4Is the start of the Scotch, but if black did something besides e5, would white still move Nf3, then d4, or would he have to do something else to match black?Thanks for any help