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Play the orthodox lines. Continue with your plan. If someone plays g4 at you, and you play the sicilian, play the sicilian.  

Remember: 

If someone surprises you at their move, don't panic. Take a deep breath.

   Just sayin.

A simple answer "may" be, evaluate the position that has been reached....you should be able to formulate a plan based on that?  i.e.  Is there some weakness you can pile up on?  A poorly defended piece due to bad placement?  Or maybe even something simpler than that...An open file?  Put your R's on it!  A plan doesn't have to be a master piece, just a simple 1, 2 move is sometimes enough.
And if none of the above applies, a plan should probably be just improve your worst placed piece.

Hope that helps a little.

(Unless I completely misunderstand what you are asking???)

Follow the basic rules of development, and you won't have difficulties against most unusual moves.

Part of the reason people play weird lines is to rattle you.  Don't let them.

When I have that happen I just dramatically swipe the pieces off the board and drink.

 Computer chess has ruined the game that way.

suppose I have white and my opponent is playing 1..Nf6 2..Ng8 3..Nf6 4..Ng8 etc just to test me.  I will try to place my pawns on e4, d4, and c4, knights on f3 and c3 lightcolored bishop on d3 and castle. Now depending on the position at this point, placement of darkcolored bishop and rooks can be decided by white.  Try to adhere to the underlying prinicples like strong central pawns and rapid piece development rather than going by the openings names and their move order.  Unusual opening moves usually concede some advantage to the other side, so rather than panicking I think we should try to take advantage of such moves