"Never let your characters relax or feel comfortable in a scene. Remember that people are not always entirely rational, especially in stress situations. If you're character's "craziness" seems in character, consider allowing him to blow up or make some stupid mistake. Your story people -- even in the toughest scenes -- are not wholly logical robots. Show clearly that the viewpoint character considers the oncoming scene as vitally important. Have him say so, or think so, or both.
Never allow a lead character to enter a scene with a lackadaisical attitude."