Although the play has no evidence for it, I like to think of Macbeth looking out over the castle walls at Birnam wood and reflecting.
Much in the way I have been reflecting on my tumour.
I can actually see it - the doctor cut a 'vent' so he could take the biopsy and has left me a nice view of the problem. For those morbid enough (or with the stomach) it looks a little like a walnut - or half a walnut (the doctor, struggling for the correct English comparison, described it as a cauliflower - which is much too floral). It reminds me of my time in school when we dissected things - and the vengeance of the mouse is upon me - very mouse brain with alcohol dripped on it.
Anyway, when you look at 'Birnam Wood' you see your fate - you have to face that fate, there is no alternative. You are looking into the probable end. Macbeth's initial reaction has to turn cold. It is not depression, it is not even depressing, there is a satisfaction as you look and reflect - a knowingness - of being tricked into a false security and of smiling at your own gulability.
There is a determination too - OK, you've got me, but I'm going down strong. Not necessarily fighting (although in Macbeth it is that), not necessarily raging - but with whatever strength you can muster and with at least an attempt at dignity.
There is a certainty of defeat - and this is possibly an English thing - fighting the game to take part, not to win. The battle will be the final one, and Macbeth has no illusions as to silly schoolboy ideals of glory - he has fought before and knows the hacking of limbs, the bathing in blood the agonies and screams - as I know the fight against pain of cancer - I've seen enough of my family fall to it not to have any illusions: There will be no dignity in the end, just the coughing up of blood,the manual evacuation and the balance between enough pain killer and not extinguishing life.
There is no bravery possible, you have a choice of illusion or facing the fight - bravery is when you choose to fight when you do not have to - Macbeth has to, I have to.
I am aware though of Macduff - Macbeth as he looks at Birnam knows nothing of him. I know there will come an energy sapping moment when defeat stares me very closely in the face. Macbeth, ever the soldier, 'lays on': I hope to goodness sake I can do the same.