Every time that I have seen a porn star, after they dance, they spend some time posing for polaroids with people (you can't take your own pictures). The level of explicitness of the poses is dependent on both the girl and the club. I never asked for boxcovers to be signed, but I would guess that if you bought some polaroids, she would also be willing to sign a cover or two.
The level of interaction with the porn star will depend a lot on the girl and the size of the crowd. I've had some long conversations with girls (Rebecca Wilde sat with me for an hour or so at a virtually empty club about 10 years ago) and I've had situations where it was like an asembly line (collect the money, take the picture, sign it, "Next").
There are also girls who will do lap dances with customers (this also depends on the club).
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