The key when danger enters a story is not to let it drag on too long, for your reader will become disinterested or frustrated. Neither do you want the danger to go away too quickly in most cases, because while your characters are in danger you have the reader's utmost attention. The ideal duration must often be learned by experience. Have you ever watched a movie wherein the duration of a dangerous situation led you to believe you had better find something else to watch because your body didn't have enough adrenalin to keep up with it for so long? Even horror flicks know the trick to a good tale is to have quiet, safe pauses between the more active pieces of script in order to allow viewer adrenalin to revamp. The mind shuts off if there is no letup.
If your endangered characters are saved too quickly or too easily the audience may give you a well deserved yawn. Draw it out a bit. Play with it. Cat and mouse it a little. Duck and dodge, fight and fight again, keeping in mind that you must neither push too much nor let go too soon.
And that is all. If I were to go on, I might achieve a well deserved yawn. Write on!