Write a single sentence that uses conflict to create suspense. It should be a situation affecting your protagonist.
Now add a sentence that starts a clock. Whether the clock is explicit or implied, it should give your protagonist a limited time to act before something happens.
Finally, add a sentence that turns the suspense into action. The action is not what happens to the protagonist; that would end the suspense. The action is what your protagonist starts to do; that continues the suspense.
The reason for using just three sentences is to show how each one serves a specific purpose in the suspense structure: conflict, clock and action.
conflict – Just before the line went dead, the voice on the phone said he would come for me after dark.
clock – The shadows grew longer and the lightÂ changed, clear and soft the way summer evenings feel just before the sun sinks below the tree line.
action – I plodded down the cellar stairs to sit with my back against the damp wall, loaded gun in my lap.