Marshall, an older colonel, interviewed hundreds of Airborne soldiers during the campaign in Normandy. He notes down all the information for future reference. He interviews Hartsock which is the story of Earned in Blood.
He then later interviews Corrion, which confused alot of people that Corrion would be the main character of Brothers in Arms: Hell's Highway. S.L.A Marshall is one of the few characters never seen in action. And so far, he only appears in Earned in Blood.