I'm not sure if there was a criterion for "Boardwalk Empire." So far I have been in a speakeasy scene, and an awards ceremony in Enoch "Nucky" Thompson's Catholic Church. I can't say too much more until the episode aires. I was both in the congregation, and later reception. Sitting in church, cross-overs for the reception. The church, by the way, was in Brooklyn.
The speakeasy scene was even better - I sat right next to Nucky's table in the speakeasy. I do have at a small chance to be seen in the scene (but you can take nothing for granted in film and TV).
The Players Club next to Gramercy Park in Gramercy was the location.
For my role, I bought the glasses at Mr. Fun's, but despite their incredibly low cost (about $3.00), they were very good for the period. I had my hair specially cut for authenticity, and it was re-cut on set by make-up ("We can't use Ohio haircuts here," I was told). My hair was then glued down with "cement." It was as stiff as a wire brush.