Shakespeare's Act 5 is full of suspense. The most effective way of achieving suspense is the short scenes that alternate between Macbeth in the castle and the gathering of forces outside the castle. We see Macbeth's fear, determination, and ultimately courage as he becomes more aware of what we already know: thousands of men are ready to do battle against him. As the scenes alternate in a fast paced way between Macbeth's growing realization of the ominous developments and the joint forces of Malcolm and Siward and Macduff, the readers are anticipating a final showdown, and the final fight scene does not disappoint.
Related to the shift in scenes is Macbeth's repetition of the prophecies that we begin to see developing in somewhat unexpected ways. When we first learn that the combined forces are meeting in Birnam Woods, we are wondering how the witches' prophecies will be fulfilled. Then, we learn that Birnam Woods will come to Dunsinane as each soldier uses a bough so that the strength of their force cannot be discovered. We already know that Macbeth should beware Macduff who is seeking vengeance for his slain wife and children. And we are waiting to see how the last prophecy will be fulfilled: "No one born of woman shall harm Macbeth." Macbeth repeats this last prophecy several times--when he kills young Siward, for instance. It is not until Macbeth and Macduff face each other, that we learn that Macduff was not born of woman. This surprising information leads us to anticipate the ensuing fight between the two.