Sets and Costume Design:

If you can afford a luxurious set, have one—if not, do what you can by suggesting it.

Animated projections mixed with practical pieces could be gorgeous.

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Women's fashion in the early to mid-20s saw a shift from Edwardarian style and elegance - a shift embraced by characters like Mary, but certainly not by Emily and Evie and that crowd.  Men's fashions were what they were, with Poirot always extra dapper.