Source: richmondbridgeassociation.org
Some opening leads are relatively easy. If you’re on lead after the opponents bid 1NT-3NT, for example, the old guideline of “fourth down from your longest and strongest suit” works well on most hands. You also have an easy lead if partner has overcalled a suit, or when you have a suit with a strong honor holding (KQJ, QJ10, AK, etc.).
On many hands, though, your choice won’t be as clear, and that’s why opening leads are one of the most difficult parts of the game. Making a good one requires require careful analysis of the auction. On some difficult hands, you’ll want to make a passive, safe opening lead that isn’t likely to give away a trick — such as a lead from a “topless” suit like 87643 or 10982. On other hands, it will pay to make an aggressive lead, such as an underlead of an unprotected honor.
How do you know when an aggressive lead is the best choice? One of your strongest clues comes when the opponents have an auction that identifies a long, strong side suit that can be set up as a source of tricks. For example, suppose you’re North holding: 
A5 
743 
KJ953 
Q72 and it’s your lead after your opponents have the following auction:
| West | East |
| 1 |
2 |
| 2 |
4 |
Your best opening lead is probably the 5 of diamonds (fourth best). You hope partner has either the ace or queen of diamonds, but even if he doesn’t, your risky lead may not cost anything. Dummy has shown a long (and probably strong) heart suit that may be used to pitch declarer’s losers. It’s important to set up possible tricks for your side right now, while you still have the trump ace as an entry to cash them.
How about leads to higher-level contracts? When the opponents bid a small slam, your natural instinct may be to make a safe opening lead, but on some hands, being passive can give away the contract. An aggressive lead is often your only chance to beat a slam, especially if the opponents have shown great strength and/or a side fit. For example, suppose your opponents bid to 6 via this auction:
| West | East |
| 1 |
2 |
| 3 |
4NT |
| 5 |
6 |
And you’re on lead with 109874 762 A3 K102.
The spade 10 looks safe, but it doesn’t rate to set up a trick for your side. That would require partner to have the spade king and dummy to have the ace (or partner to have KJ and dummy the queen), and declarer and dummy to each have at least two spades. It’s better to set your sights lower and play partner for the club queen. Lead the club 2 and hope you can set up and cash a club when you get in with the diamond ace. If it happens that you’ve led into declarer’s AQ of clubs, you may have lost nothing, since it’s likely that his potential club losers would have been pitched on dummy’s diamonds.