How was Laurence Olivier able to find a cast of thousands in 1944 for his film Henry V, particularly the Agincourt scenes, when surely every able-bodied man ought to have been fighting the current enemy?
IMDb has some interesting trivia about this.The battle scenes were apparently filmed in Ireland and the cast of thousands were mostly servicemen including Americans.The Government commisioned the film as a morale booster and so presumably the MoD would have arranged for the participation of the servicemen.