Here are some bidding and leading questions worth considering:

Q. 1—Sitting South, with West dealing and no one vulnerable, you hold:

♠️ K 9
♥️ Q J 4 3
♦️ Q J 8 7
♣️ K J 7

The bidding has proceeded as follows:

West         North        East        South
1♣️          1♠️            Pass        ?

What do you bid?

A.—Since partner may have overcalled with as few as eight high-card points, you should increase the requirements to respond in NoTrump to 8-11 for 1NT, 12-15 for 2NT and 15+ for 3NT. With 13 HCP bid 2NT. 

Q. 2—Sitting South, with East dealing and East-West vulnerable, you hold:

♠️ 9 4
♥️ J 9 8 7 5
♦️ A K 8 4
♣️ Q 9

The bidding has proceeded as follows:

East         South          West          North
Pass         Pass             1♣️          Pass
1♠️          Pass             1NT          Pass
2♣️         Pass             2♠️          Pass
Pass   ?

(West should have passed 2♣️, because East didn’t promise more than four spades.)

What do you bid?

A.—This is a close call, but with favorable vulnerability I would double. You probably have a fit in diamonds or hearts, and your opponents figure to hold only about half the points in the deck.

Q. 3—Sitting South, with West dealing and both sides vulnerable, you hold:

♠️ A K Q 5 4
♥️ A 8 7
♦️ K 2
♣️ J 7 3

West opens 1♣️, and North passes, and East bids 1D. What do you bid?
A.—Normally I’d double and then rebid 1♠️ to show 17-19 HCP. In this case, you can’t expect partner to hold much of anything, and the opponents will probably keep bidding and prevent partner from doing anything but passing. So I’d bid 1♠️ to give partner a lead.

Q. 4—Sitting South, with East dealing and East-West vulnerable, you hold:

♠️ Q J 9 4
♥️ 6 5
♦️ 3 2
♣️ A 10 9 7 3

The bidding proceeded as follows:
East           South           West           North
2NT           Pass             3NT           All Pass

What would you lead?

A.—I wouldn’t blame you for leading the ♣️10, but there’s something to consider. East-West didn’t transfer or use Stayman, so it’s unlikely they have an eight-card major-suit fit. Accordingly, I’d opt to lead the ♠️Q.