The opening scene where the vets in the hospital are talking was totally unscripted. They were real Vietnam vets discussing their own views about the war. Jon Voight was supposed to have added to the dialog, but out of respect stayed silent and listened.
One of the first films even partially dealing openly with the idea of sex between an able-bodied person and a disabled one. Other films dealing with disability and romance had indirectly implied it or avoided it altogether (such as in The Best Years of Our Lives (1946)).
Jon Voight's role of Luke Martin was loosely inspired by paralyzed Vietnam veteran Ron Kovic, who was making inroads in Hollywood with his book "Born on the Fourth of July" at the time. Of course, Kovic's book and story was in 1989 put on the screen with Tom Cruise in the role of Kovic.