You're imagining someone genuinely doing cosplay, not someone, who may not be a perfect actor, trying to pull that off, and also stick to experiment protocol.

AFAIK, many (most?) people can pick up on subtle cues that something is off in that person's behavior, in their expressions, body language, how and where they look and don't, etc. And the experiment intentionally draws bystanders' attention, so many of those cues will in their sight.

Perhaps this is another "further research".