Kelly Gaffney, assistant district attorney, would’ve likely gotten out of jury duty, but since she was only a TV character on the now-canceled Law & Order: Trial by Jury, the actress who played her, Amy Carlson, was stuck doing her civic duty at 100 Centre Street from October 11 through November 7. Carlson was even randomly selected to be foreman, which meant she had to swear in the witnesses in the 60 or so Manhattan criminal cases that came before her jury. “The first couple of times I approached the grand jury, I was looking around for my mark,” Carlson says. “Some of the cops looked at me funny. Then when they walked out, they’d point their finger like, ‘I know you.’ ” But doesn’t not being a lawyer and playing one on TV tempt one to shout, “I object!”? “Absolutely!” she says. “Sometimes I’d be thinking, Objection! Leading the witness! You can’t do that! Then I’d realize, Wait, it’s a grand jury. They can do that.” Next: Jean Valjean, Upper East Side Cantor