For me it depends on the type of move and position. In a open position I'd play as agressive as possible as I noticed that people who usually play those kind of moves like 2..f5 often dont like to defend and also often have bad memories for remembering tactical lines which will follow. They often just remember the moves and are stuck when you dont play exactly as they remember.
If it is a less logical move such as Nc6 in a french structure I just play similar to what I would normally do and try to find a way to exploit the move, which is not always possible. For example the Bird defense in the Ruy still exists.

How you react to rare moves or novelties in the Opening, depends heavily on how one studies Openings in the first place.

Some amateurs choose to memorise their Openings by rote, and such a superficial approach tends to let one down when faced with an uncommon or not well known move. 

I have stressed many times that aspiring players must strive to understand thoroughly the themes and ideas behind the Openings they play, as only in this way will they be better equipped to successfully deal with a new idea or move over the board. It is also indespensible to understand fully the rudiments of what is considered sound Opening play, such as: Control of the Center; Not moving the same piece twice; Developing all of your pieces; King Safety; Don't bring your Queen out too early; Don't go pawn hunting; Connect the rooks; Knights before Bishops; etc etc etc

Once one has a firm grasp of the rudiments of good Opening play, the next thing an aspiring player quickly realises is that these rules though sound in general are not meant to be followed blindly or dogmatically. That is to say that through experience your judgement should devlop to the point that one knows when some of these Opening postulates can be bent and sometimes broken.

The great Cuban Jose Capablanca once challenged the rule of moving the same piece twice in the Opening, by stating that it was fine to do so, provided that the move carried a strong enough threat. This is one of the various dilemmas an aspiring player is faced with on his journey towards master level.  

A non chess player once asked me "What is the fundamental difference between two players once they have reached master level" his point being that surely every master understands fully the tenants of good Opening, Middle and Endgame play, so how is it then that one master can be so much stronger than another? My answer was that among other things, a chess battle between two masters often does not revolve around good ideas verses bad ideas but rather good ideas verses better ones.

In summary, seek to understand the Openings you play rather than memorise them and you should be able to better cope with surprises at the board.

Tops  

 

I would second the recommendation of John Emms's book.  He has very quietly become one of my favorite chess authors.  I'm trying to think of a book of his I haven't like.  He stresses clarity and explanation, and at the same time doesn't skimp on quality analysis and ideas...

And these Danish lines look very interesting.  I'll have to give these a look...

bravehoptoad, that's pretty much what I do, maybe for a different reason, it ends up being the same. I always decline gambits. I play 1. e4e5 2.Nf3Nc6 3. d4 exd 4.c3d3!? and 1. e4c5 2.d4cxd 3. c3 d3!? 

I am a better attacker than a defender, so I try to reduce my oponent's attacking chances as much as possible. Also declining a gambit reduces possiblity of falling into the trap. So I kill two birds with one stone here. Except for the objectivety part, of course.

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Against many rare moves, the same strategy that works in main line and 90% of other moves in that line won't work, that's why people would play that. Maybe I am missing your point, though...

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I guess, if I had seen this, I wouldn't have any problems . The hard part is seeing that, and it might look like you see it, but that's a trap!

At my level, every move is a rare/novelty 8)

Thanks for the reply,

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I like this way of thinking!

Unfortunately, I have some kind of paranoja about walking in into somebody's opening trap. It's like losing to somebody who hasn't even thought at the board for a minute. 

I suppose it's where this comes in Quote: