The characters of Macbeth can be shown in many different ways. Here are my picks for how I imagine cast based on Act 1.
King Duncan
I chose George Clooney as my actor for King Duncan because I see him as old and serious, has facial hair, but very well put together as a King. King Duncan is involved with his people, very merry, and makes strong decisions telling me that he has an old wise look to him and he has had many life experiences. He is old enough to be deciding who will inherit the throne after him but not yet to step down himself.
Macbeth
I chose Brad Pitt as my actor for Macbeth because I see him as young but aged enough to be married, rugged looking from battle; with longer hair and facial hair, but also handsome enough to be charming the King so much. He is a soldier in war, but is not one for killing who he follows so he has a visible softer side with less harsh features.
Lady Macbeth

I chose Emilia Clarke for Lady Macbeth because she has visible beauty, but also has an aggressive tone to her. I see her with white hair because I believe it suits the time period and being generally a color for elderly people, it can make her look more wise or unique to stand out. She has a high feeling of determination and control, making me choose an image with a more fierce look.
Banquo
I chose Zach Galifianakis for the role of Banquo because like Macbeth, he is also a soldier, giving me the image of a build up of facial hair from being away. I believe he is a shorter, humble man who is loyal to his friend and easy to be around.
3 Witches
For the 3 witches, I chose Angelica Huston, Eva Green, and Miriam Margolyes. I believe that each should have a different sort of role contribution to the group in this plot, instead of each being the stereotypical witch that is ugly with a big nose. The first, mainly the leader, with harsh features and is very strong. The second being more attractive but still with a dark aspect to her looks. The third that is clumsy, but is older with more wisdom.