We all have our favorite openings.  Ones that we have studied thoroughly and have played dozens of times.  Whenever one of our games begins with that opening, it evokes warm memories of brilliant wins from the past and naturally gives us a feeling of invincibility, since we are "playing in our own backyard", so to speak.  As we trot out our book moves, we hope our opponent plays lines we know to be inferior, so we can show off our deep reservoir of opening knowledge and quickly gain the advantage, if not an outright win.

Occasionally, however, our opponent plays a move that we have never seen before.  At first, we are insulted by his impudence: "That's not a move!", we yell without speaking.  "I know this opening inside out and I have NEVER seen that move, so it CAN'T be any good!"  But after the initial contempt for the move fades, we get down to the task of formulating our required reply.  Can we just ignore it and continue to play as we usually do against the "correct" move?  Is it a trap?  Does it contain some sort of threat?  Or is our  "uneducated" opponent merely unaware of the motifs of this opening and is simply "making it up as he goes along?"  As we continue to analyze the move, we notice that perhaps it does have some point to it.  It may allow him to persue ideas that are normally not available to him in this opening.  Maybe he has studied this line inside and out.  Maybe (perish the thought!) he knows more about this opening than we do.  Maybe HE has scored dozens of wins with this opening.  Maybe we have entered HIS backyard!

All of these emotional undercurrents can cause us (and even Grandmasters) to react poorly to such a move.  That is why it helps to have a mental checklist onto which we can fall back when we are confronted by a new move in the opening.  Such a checklist will help us clearly evaluate the pros and cons of the new move and come up with an appropriate reply, rather than lashing out with the first move that pops into our heads.

Checklist For Replying To A New Move

1.  Does the move threaten something immediately? 
A check? A capture? A fork?  If so, this threat needs to be dealt with right away.  Don't miss anything obvious just because you are surprised by a new move.

2. What is the normal move in this position and what is it's purpose? 
Does the new move accomplish this purpose as well, or does it allow you to play a shot that is not normally possible?  Sometimes this elementary question can instantly lead you to the correct reply.  For example, if the normal book move prevents you from playing a very unpleasant pin that you were threatening and the new move does not, maybe you should just play the pin and gain an immediate advantage.

3. Is the new move designed to prevent you from playing a key move in normal line?
 Sometimes the purpose of a new move is merely to prevent you from playing a string of book moves and to steer you into a new or less well known position.  This new position may be objectively worse for your opponent than the main line, but he is hoping to throw you off.  In this case, there must be a reason that this new move is not the main line.  Perhaps it takes too much time.  If the new move is a pawn move, maybe it leaves you too far ahead in development.  If the main line is for him to castle, maybe the new move allows you to open lines and attack.  Instead of trying find out what the moves DOES, find out what it DOESN"T do.  In other words, sometimes you need to look at the hole instead of the doughnut.

In the following game, White's fifth move comes as a surprise to Black.  Not seeing any particular threat in the move, he continues as per the normal line.  After making some preliminary arrangements of his pieces, he seeks to exploit a perceived drawback to the new move.  White plays somewhat passively and allows Black to build an attacking position, which, with the help of some tactical strokes, crashes through for the win.

The lesson here is that when you are confronted by a new move, you need to:
1) Remain calm and don't respond quickly or emotionally. 

If you think responding quickly will upset your opponent by making him think you have seen the move before, or you think that taking time to analyze the move will encourage him by letting feel as though he has stumped you, you are wrong.  A quick, dumb move will encourage him, but a well thought out move and follow-up plan will upset him.

2) Try to find out what the new move is designed to do.

Any move, even ones that are proven to be bad by good play, usually has some threat and will be effective against poor play that does not respond to that threat.  Don't fall victim to simple traps just because they are new to you.

3) Try to remember what the purpose of the normal move is.

The normal or book move is usually the book move because it is thought to be best in that position.  Other moves can range from game-losing blunders that can be defeated in one move to mere variations in style, with effectiveness equal to the book move, but requiring a different plan to combat.  Don't dismiss a new move, but don't fear it either.  Calm analysis will guide you to the best reply.