I read War and Peace recently and of course there is the part during the Battle of Borodino where Pierre Bezukhov, a pretty much random person shows up and starts fighting French people and getting ammunition and what not. Was this just a device for the book or would random people show up and fight frequently?
You might be interested in this account of the first battle of bull run. When I was younger I read about the myth of sightseers, and it is always interesting to relook at a subject one thought one knew, but this is a good example of people a hundred and fifty years ago watching a battle.