The old man at the bridge is just an ordinary guy from a small town. He's not actively involved in the Spanish Civil War; he doesn't have any deep-seated political convictions. And yet, he's had his whole life disrupted by the rapid spread of this bitter, bloody conflict. The war forced him to move from his home town of San Carlos, where he used to take care of animals. And now, as he sits by the side of the road in the dust, he has to move on yet again if he's not to fall into the hands of marauding Fascists.
But despite the imminent threat of danger, the old man is unable to get up and join the thousands of other ordinary people who've also had their lives disrupted by war—who are making their way across the bridge to safety. He's been so psychologically damaged by his experiences of conflict that he's pretty much given up the ghost. He no longer seems to have the will to live.