When I was in hospital with pneumonia across from me was a WW1 veteran, old, frail, barely moved never spoke a word.
Middle of the night he would wake up, sit up shouting in a voice that could still have made a private nervous, "The bastards, man the guns, their coming, wait for your shot, flares! flares! get those bloody flares up". And then the nurses would shush him down and he would be back to frail old man. Poor bugger died while I was in there, just quietly passed. At the time 80 years on and he was still having nightmares.