A year ago, I applied to a Craigslist want ad and got the call: “Will you accept playing the part of a young Depression-era housewife in the Johnny Depp gangster melodrama Public Enemies?” Will I accept? “How much can I pay you?” was more the question. And so my experience as a background starlet, er, extra in this summer’s blockbuster began. Left: In costume on the Public Enemies…