1) Do not ask them, as I tended to do, about the contradictions between their transportation to the event (cars and trucks, including the one I photographed above) and their belief that they were being completely authentic. Much more fun to instead ask for their cell phone number so you can call them later if you need to clarify anything. Not only did many give me their cell number before realizing I was going to mention that in the story but more than one gave me their business card.
One of my favorite reenactment stories that I wrote was about a hospital guard who went around the country, by plane of course, appearing as Ulysses S. Grant. Not only did he have a business card but he also had an email address and Web site. I found my lede for the story in noting that Ulysses Grant is a guard at the local hospital but struggling to get his Web site working, which is about what one would expect if Grant were alive today.
2) Do not call them actors. They prefer the term reenactors It's a bit like the Elvis impersonators, many of whom I've also interviewed, who refuse to answer to that label declaring they are "tribute artists." It may be semantics to you and me but not to them. If you want to read a great book about re-enacotrs read Confederates Of the Attic by Tony Horwitz.
3) Do not ask this question I asked in July 1999 of those playing the confederates: "How do they feel about the objections people have towards them waving and using the confederate flag?" I think I also pointed out that some would see their actions as implying some form of racism.
One guy started to talk, saying those who suggest the Civil War was about slavery are idiots, ignorant, full of baloney, etc. He was getting a bit riled up. I asked politely for his short answer, reminding him I had limited space, and his colleagues began to surround me.
I noticed then, for the first time, that they were indeed holding muskets. "Those things aren't loaded, are they?" I asked innocently, writing down notes. "Usually not, but one guy got shot with a bullet in Gettsyburg last year² one replied. I quickly ended the interview.
4) Avoid asking them if in order to be truly authentic they each have their own set of slaves.
5) Lastly I'd advise not asking whether the camaraderie they get as reneactors is similar to that members of gangs have. I asked once and the reenactor took it completely the wrong way.