The Dummy is on LSD?

When you see the dummy, either literally or in your mind’s eye before the opening lead, it will be either Long, Short, or Dead. LSD!

Long means the dummy has a long suit that is dangerous, either in and of itself or because you know finesses will work and it will run. We must be aggressive and take tricks or set them up quickly. Break suits to take tricks or set them up.

Short means the dummy is short in a suit and declarer will be trumping those. Lead trump if you can cut the ruffing power. Or perhaps you know a cross-ruff is coming, time to get our tricks if we can’t strip some trump. And finally,

Dead means the dummy has only HCP to contribute, and we can be patient and wait for our tricks. Play passively. Don’t idly switch suits.

Classify the dummy and decide your strategy accordingly.