An excellent and intriguing question. I’ve put a good days thought into this. I’m going to answer with several things in mind. 1. The party is happening in 1939, so I haven’t thought of any modern day costumes. As thrilling as it would be to see Hecate in a crazy GaGa inspired meat dress, I have strived to keep the costumes in period. 2. I am picking costumes mostly based on characters and not on the actors performing the roles. Certain performers have a greater influence on me in certin roles, and I couldn’t help but let that inspire my choices, but I will be listing by character and not performer. You can read whatever you want into who you think I’m thinking about. With those two things being understood, here goes!
1. Macbeth - Very obvious, but Macbeth would come dressed in full royal deckings as a King. Complete with crown and scepter, he would flout his ambition and desire in everyones face.
2. Lady Macbeth - Good old Lady M would come in a straight jacket. I must say that I was inspired by Alan Cummings broadway performance for this costume, but I feel like of the two of them, Lady Macbeth loses her sanity, while Macbeth simply loses his life.
3. Macduff and Lady Macduff - The Macduff’s would come together as one of those two person horse costumes. Its important to know that Lady Macduff would be the head, as most of the time, the people wearing this costume seem to be moving awkwardly, almost as if drunk. Just as important, Macduff would be the rear (not implying he’s a horses ass) simply because he always seems a few step behind his wife.
4. Banquo - In a grand foretelling of things to come, Banquo arrives at the gala in a simple white bed-sheet with two holes cut into it. Banquo would definitely throw his sheet on quickly while no one but Macbeth was looking, and quickly tear it off as soon as Macbeth tried to point him out to everyone else in attendance.
5. Duncan - I see Duncan coming in as a clown, but not the traditional happy clown. More like the black and white sad clown a la Puddles.
6. Malcolm - Malcolm would come as a knight in a full suit of armor. It takes a lot of endurance to trudge around in heavy armor, and Malcolm has endurance to spare.
7. Sexy Witch - The witches would definitely conspire to come together as a cohesive three person team. The Three Stooges were at the height of their popularity in this time. Sexy would go as Curly.
8. Bald Witch - Bald witch would be Moe, the leader of the stooges. This way, she would get to wear a wig, and make her other witchy cohorts deal with male (or female) pattern baldness.
9. Boy Witch - And Larry is the only Stooge left (excluding Shemp of course). It wouldn’t be too hard for boy witch to shape his ginger hair into Larry’s trademark coif.
10. Hecate - Add a little tail and horns to that red dress, and Hecate transforms herself into the perfect little devil. She already does such devilish things, that wicked woman.
11. Porter - Porter would come in one of those fancy, half man half woman get ups. Porter would shave half of his face, apply a wig and makeup on that one side and wear a half dress on that side as well. He’d go around the party having conversations with his two sides and only ever speaking to boy witch with his female persona.
12. Catherine Campbell - Believe it or not, Catherine would come as a maid, albeit, a sexy french maid. Not one to leave her duties behind, she would still insist on working at the party, but would change her look only minutely. And of course since Duncan is throwing the party, the french maid outfit would be perfect.
13. Speakeasy - Speakeasy would come as prohibition era gangster Al Capone. Having run a speakeasy, Speakeasy is all too familiar with the illegalities of selling and procuring alcohol at the time. Capone ran his syndicate mostly during the years of Prohibition (this one is a stretch, but it was pretty hard to come by)
14. J. Fulton - Fairly obvious, but Fulton would arrive in full priest regalia. He may even dress like the pope. With crucifix and bible in hand, he would hang on the sidelines, never fully engaging in the festivities.
15. Bargarran - Easily one of the creepiest in the room, Bargarran would wear an overcoat, and the illusion of little to nothing else. Would he be clothed underneath, or really take on the role as a flasher extraordinaire? That will be for the other guests to find out.
16. Nurse Shaw - By far the hardest for me to think of (I haven’t spent much time with our dear nurse) the Nurse would throw of her uniform and don the garb of a construction worker a la “Rosie the Riveter (even though this wouldn’t be popularized for three more years) Why? Because whenever she needs a nail to hammer in, she can just cough one up!
17. Matron Lang - Matron looks to literary influences and comes inspired by Charles Dickens. She dresses as Miss Havisham from Great Expectations(1861) the shriveled and bitter old spinster who lives in the attic. Not a very far stretch.
18. Agnes Naismith - Agnes would come in a deerstalker cap and emulate Sherlock Holmes. Highly popularized in stories and movies of the era, Sherlock is the perfect persona for Agnes, as she would be searching the grounds for clues as to the disappearance of her sister Grace.
19. Annabella - Its a shame that Poison Ivy from Batman fame wasn’t around in 1939. But along the same idea, Anabella would be a wood nymph, bedecked with greenery and flora.
20. Maxamillian - Since you specifically mentioned Max, I’ll give him a costume, but it’s too time consuming to go through every single Man and Woman in bar. Maxamillian arrives as a trainyard hobo, complete with a stick and bundle and an oversized bottle of real gin, which he would swig at with alacrity.
Hopefully I haven’t missed anyone. Everyone, feel free to agree or disagree with anything I said, or correct me on any anachronisms I may have put into place. Keep in mind, this is only my opinion. Also feel free to add any suggestions or ideas that you have on the subject. And again, thanks to this particular Anonymous for the question.