Finally watching Black Swan and this early scene puzzled me. While Lily taking off her knickers isn't completely bizarre, she does it around someone she's never met. And there is no follow up of any kind. No scene of Lily with someone at the gala. Why does she do that? Was there any cut scenes or director commentary on this scene?
So much of Black Swan is left open for interpretation. I always thought that Nina perceived Lily as a threat in all aspects of her life. Lily removing her panties just reiterates the point that Lily has no inhibitions and Nina has several.
Nina might just be imagining Lily removing her panties. This movie can be interpreted on so many different levels. I am just giving you my interpretation.
No sources to cite due to viewer interpretation, not fact based.