Stephen Rodrick was on set to interview Michael Winterbottom for the New York Times magazine (published on 3 July 2005) when he was roped in to appear in the film.
All of the armed soldiers (those with working guns) in the battle scenes, plus some other extras were members of the Sealed Knot Society, which tours the UK, performing historical re-enactments of the English Civil War.
The film also features several pieces of music used by Stanley Kubrick in similar situations in Barry Lyndon (namely the Sarabande by Georg Fridrich Händel and traditional British Grenadiers), a film set in about the same location and period.