In addition to what other contributors have already stated, I would further suggest that, if you are asked to write a monologue for the doctor, you think carefully in terms of the larger context in which this monologue would be set. At what point, within the course of this play, would this monologue take place, and, within that context, what would we expect to be on the doctor's mind at this point in time?

Even in the doctor's case (minor character though he is), within the context of that fictional world, we must assume he has a larger character arc of his own (even if we see only a very small portion of it). Thus, if you write a monologue which would precede the events of act 5, scene 1, before he has witnessed Lady Macbeth's madness, he would have a very different perspective than were it to be set after that point. Likewise, if the monologue was set during the height of the battle between Macbeth's forces and Malcolm's, you would have to expect his mindset would be very different.

First, by this point, he would have witnessed Lady Macbeth's madness, and thus, he would have gained an awareness of Dunsinane's dark history. Moreover, at this point he would also be in a situation of very real uncertainty, both as far as it relates Scotland's future but also with potential danger to himself. Finally, if you were to set this monologue immediately after the battle had been won and order had been restored—or in a still more distant future, with the doctor recollecting on this past history from a point of far greater distance—that too would imply a very different context and would need to be written accordingly.

To conclude, it might be worth keeping in mind that monologues have a dramatic component to them and are inevitably impacted by the events and context which would surround them at any particular point within the course of the play.

To write a monologue for the Doctor, one must be familiar with his character in Macbeth and have insight into his thoughts and feelings. In act five, scene 1, the Doctor speaks with the Gentlewoman regarding Lady Macbeth's odd behavior at night. The Doctor then watches as Lady Macbeth sleepwalks at night and acts like she is attempting to wash the imaginary blood from her hands. As the Doctor and the Gentlewoman witness Lady Macbeth sleepwalking, they hear her speak about the "old man" (King Duncan), the murder of Macduff's wife, and Banquo's death, which reveals the dark truth that Macbeth assassinated the king and other prominent people in Scotland. The Doctor immediately recognizes that Lady Macbeth has a heavy heart and is filled with guilt by saying, "What a sigh is there! The heart is sorely charged" (Shakespeare, 5.1.35). The Doctor then indicates that he cannot help Lady Macbeth by saying, "To their deaf pillows will discharge their secrets. More needs she the divine than the physician" (Shakespeare, 5.1.51-52). After witnessing Lady Macbeth share her dark secrets, the Doctor recognizes that she has a guilty conscience and dares not to speak about what he saw and heard.

Using the information presented regarding the Doctor's experience witnessing Lady Macbeth sleepwalking, one could write a monologue expressing the Doctor's overwhelming emotions regarding Lady Macbeth's behavior. A monologue for the Doctor could include the difficulties he faces knowing that Macbeth murdered King Duncan, Banquo, and Lady Macduff. One could also address the Doctor's fears of living in a dangerous environment under the rule of a murderous tyrant. The monologue could also address the Doctor's thoughts regarding Lady Macbeth's soul and her need for spiritual healing rather than medical help.

If you were to write a monologue from the doctor's perspective, it would be vital to include his most important insights. These insights come when he first observes Lady Macbeth to be sleepwalking in Act 5, Scene 1.  He has come at the request of her gentlewoman who has seen her repeating the same disturbing motions and the same disturbing words for many nights now. 

Having heard Lady Macbeth ask, "who would have thought the old man / to have so much in him?" and "The Thane of Fife had a wife.  Where is / she now?  What, will these hands ne'er be clean?" he realizes that her "heart is sorely charged"  (5.1.41-42, 44-45, 56-57).  It does not take a doctor to gather that Lady Macbeth has some terrible weight on her conscience; her gentlewoman has already ascertained as much.  The doctor realizes that "This disease is beyond [his] practice" (5.1.62).  In other words, he knows that Lady Macbeth does not require a physician, as it is not a physical ailment from which she suffers; she could make better use of a priest because her trouble is clearly spiritual.  Moreover, the doctor says that "Unnatural deeds / do breed unnatural troubles.  Infected minds / to their deaf pillows will discharge their secrets" (5.1.75-77).  He recognizes that the only thing that could have distressed her so greatly is something that goes strongly against her conscience because people who are disturbed by these kinds of troubles will always talk in their sleep.  Any monologue of the doctor's should absolutely make reference to all of these revelations.