It is one of the basics of bridge that after the auction one pair defends while the other declares. However, many contracts are won or lost simply because the players did not listen to or remember the auction. See how you would have done with these hands.
PLAY: The only thing North remembered from the bidding was his partner’s double so he cooperated by leading a diamond against the 6 contract. Declarer won the diamond lead, lost a trump trick, and quickly claimed twelve tricks. If North had only LISTENED TO THE BIDDING, he would have realized that declarer bid clubs twice to show at least five, dummy jumped to 2NT indicating a balanced hand with at least two clubs, so it was simple math to realize South could hold no more than one club. Holding the trump ace, it would be a simple matter for North to lead a club, win the spade at trick two, and give his partner a club ruff to defeat the contract.
BIDDING: South could have just bid 2 but the jump to 3 in the pass out position shows an intermediate hand with a good suit. North liked the location of her spade king and continued on to game.
PLAY: West led the king of clubs and declarer saw that he had four possible losers: one spade, two diamonds, and one club. From the BIDDING, it seemed likely that West held most of the high cards but there was room for the spade jack or diamond queen in the East hand. Since it was important to never let East in to lead a diamond through, declarer ducked the first trick. West continued with a club and South drew trumps in two rounds while ruffing his last club. South then led a low spade that West had to duck. Winning with the king, he continued with a small spade toward his hand. East had to play the jack, which solved declarer’s problem. However, if East has followed with a small spade, declarer still had West in trouble. If West led a spade or a diamond, declarer would have ten tricks. If instead he led a club, it would allow declarer to ruff in dummy as he discarded a losing diamond. It came down to placing the high cards from the bidding.