Brookies are all I fish. They can be very tricky at times. If the water is fished hard any one day, brookies usually need some time to recover. To get over disturbance of fishermen tramping through the pools stirring up the bottom and such. There is no "too careful" when approaching the pool. To get them back rising they may need a few days of peace. Which is sometimes hard to find with so many warriors wading up and down the streams and rivers.
Stay out of the water as much as possible. Floating debris will spook them as will snapping branches underwater or grinding and knashing of stream bed rocks. Slow down, they are very sensitive to movements in and above the water. No big body movements. One poorly presented fly cast can ruin a pool for an hour or so. I have found fishing downn stream a little better as you can float and drift the fly into their zone on the current. But approach with caution as sound and debris travels down stream too.
If you see fish darting up and down past you. You can forget it for a while. Spooked fish can trigger a mas hysteria. One fish spooked below can go up or down to the next pool and put it's occupants on amber alert. Best advice I can give is to stay out of the water as much as possible. Approach pools from land or walking as close to the shore line as possible. If you try to cut corners getting to them they will know.
Whenever I go to a spot, I will usually sit and observe, have a puff, or change the fly, wait until I see fish rising before I wade out carefully and slowly. Even little ripples off your legs, can signal them something out of the ordinary is a foot.
Whether or not a big brookie hits your fly is ultimately out of your control. However all the stuff mentioned above is within your control. With them it's an accumulation of all the details. If you give them the respect they deserve they are more likely to strike. Think like a great blue heron. Ever watch one of them get into position? Masters of stealth and stalk.