re-enactor
A person who wishes to recreate a specific time period in history, often with the intent of educating the general public.  Some may have family members who took part in a historical event, and choose to re-enact in order to gain a greater understanding of their family member's experiences.

In the U.S., Civil War re-enacting and World War II re-enacting are considered the most popular, though other forms of re-enacting such as the Renaissance, the American Revolution, and even Vietnam War re-enacting are also in existence.

Re-enactors specialize in a form of history education known as "living history," which seeks to bring historical events mentioned in books "to life" so that those wishing to learn can get an up-close-and-personal experience.

Historically accurate clothing and accessories are a requirement to be a re-enactor, and many usually conduct their own extensive research to ensure that their historical alter egos are displayed properly.

Re-enactors are often criticized as "nerds" or "geeks" because they "dress up and run around in funny clothes and  think they're cool."  Also, some re-enactors are criticized because they tend to recreate military units noted as "legendary" (such as the 20th Maine in Civil War re-enacting or the 101st Airborne in WWII re-enacting) while ignoring more "average" soldiers.  More simply, some people just cannot understand why re-enactors do what they do.

Still, it is worth noting that without re-enactors, the cast of historical films such as "Gettysburg" would be extremely sparse.  You can't make a war movie without soldiers after all.
"My grandfather was in World War II, so I re-enact to get a better understanding of what he went through."

"Look at those re-enactors!  Haha!  What nerds!"

"If you didn't have re-enactors, the Pickett's Charge scene in 'Gettysburg' would have a cast of five people."