This applies to defenders as well. They should take the time when declarer is thinking to do some of their own. My biggest pet peeve is when a defender detaches a card before I've called a card from dummy. It's unwise and conveys unauthorized information to their partner. It says that they will have nothing to think about regardless of what declarer plays, and from my experience, conveys an emphatic signal, either positive or negative.
Enough of the soapbox, let's look at some examples:

	Tx AJTxxx Ax J8x
	AJ9x K JTxx AK7x

The opponents are silent as you wind up declaring 3NT. The opening lead is the 3. An impatient declarer would call for the J at trick one, hoping LHO led from the Q. Or they might play low and when RHO follows low win the trick cheaply with the 7. In either case, they will find they can no longer make the contract. Let's take a moment to study the hand and then play to trick 1. We'll need to bring in the heart suit to make our contract. The heart blockage makes things a little awkward. One option is to hope for the Q to drop doubleton. That will make it easy. If that happens, it won't matter what we do, the contract is secure. Another chance is to play for a 3-3 split and overtake the K and continue the suit. The A will provide an entry to run the rest of the suit. That's a higher percentage play than the first line but it's not the best. If dummy had another entry we could succeed with any 4-2 split in hearts. Look at the dummy closely, do you see another entry? Given the opening lead, the J is almost certainly a 2nd entry but declarer must resist the urge to win cheaply at trick 1. The first few tricks proceed:

Trick:

1. 3, x, 5, A!
   
2. K, x, x, x
   
3. x, ?

LHO has no recourse. If he ducks, the J will win and you can knock out the Q with the A as a re-entry. Even if he takes the Q, what can he do? You will win any return and knock out the Q as before and the opponents can take no more than 4 tricks. Be careful to cross to dummy with a club and not a diamond, if a spade is led, or you will set up a 5th trick for the defense.

Wasting the J at trick 1, removes the entry prematurely before hearts are unblocked. Now declarer must rely on a 3-3 break. Winning cheaply at trick 1 is no better. LHO can win the 2nd club and you will not have another low club to reach dummy and will again need to rely on a 3-3 break.

Visualize how the plan will go before playing to trick 1. Here's a similar example:

	Qxx Tx KQJTxx xx
	AJx AJ8x xx AJ9x

Partner has raised our 1NT opening to 3, banking on bringing in the diamond suit. A very reasonable gamble. Unfortunately the hands don't fit well, and the contract will need some luck. The opening lead is the 5. Is that good news or bad? It gives us an extra trick immediately but one we were eventually entitled to anyway. Seeing that we'll need to work on the diamond suit, we play low from dummy and win the J. We hope the opponents will take the A on the first round. No luck, the opponents are paying close attention and RHO wins the 2nd round. A 2nd spade comes back, we duck hopefully, but LHO wins the K. With no further entry to dummy, we go down several tricks. Could we have done better? Take a moment to decide before proceeding.

The answer is yes, but once again we must plan the play at trick 1. Barring an unlikely layout or extremely friendly opponents, we will need a re-entry to enjoy the established diamonds. Looking at dummy, that can only be the Q. Ironically, if our spade holding were weaker say: Axx the winning play would be more obvious. Win the first trick with the A!. Now when we knock out the A, we can lead a low spade to the Q succeeding when LHO has the king, a very likely occurrence given the lead and the defense is powerless to stop us.

One last example:

	AJxx Axxx KTx Jx
	QT9xx Jx Axx KQx

We reach the contract of 4 on a routine auction. LHO leads the 2. What are the chances for success? What is your plan? Take a moment to consider your alternatives before proceeding.

The reflexive line is to play low from dummy, maybe RHO will play an honor and we can finesse the T on the 2nd round. Plus, we'll be in hand to take the trump finesse. We'll have a chance but we can do better. In fact, we have a foolproof line, barring extreme distribution. Win the first trick on the board with the K and lead the J. We can set up a club trick to throw away dummy's losing diamond, then we can take a trump finesse for an overtrick. This line will likely fail if the opening lead is a singleton, but that is very unlikely (unless say RHO has overcalled diamonds on the hand). In practice, this line will succeed every time, or at least every time the contract can be made.

In a bridge book or column, the moment of truth is usually indicated based on how the problem is framed. In real life, a bell does not go off when it's time to make a critical decision. Make a habit, as declarer or defender, to take a moment at trick 1 and formulate your plan.