It was an aspect of unreality.

They were all so unreal, the characters in this strange drama.
Thornton Lyne seemed fantastic, and fantastic indeed was his end.
Milburgh, with his perpetual smirk, his little stoop, his broad, fat face and half-bald head; Mrs.Rider, a pale ghost of a woman who flitted in and out of the story, or rather hovered about it, never seeming to intrude, yet never wholly separated from its tragic process; Ling Chu, imperturbable, bringing with him the atmosphere of that land of intrigue and mystery and motive, China.

Odette Rider alone was real.

She was life; warm, palpitating, wonderful.
Tarling frowned and rose stiffly from his chair.

He despised himself a little for this weakness of his.