In Chapter 5 of Book 2, he gets his chance to participate in battle and comes to the realization that he is a coward. "The fear of death and of the stretchers, and love of the sun and of life, all merged into one feeling of sickening agitation" (128). This also seems like a natural reaction, and it makes sense that an untrained soldier would be shocked and afraid when seeing the reality of battle for the first time.
My main interest in Rostóv comes in Book 3, especially in Chapters 9 and 12, where we see him take a new attitude, inspired by the Emperor. He seems to have overcome his fear of death and is now willing and even eager to die for his Emperor. "What happiness it would be to die - not in saving the Emperor's life, but simply to die before his eyes" (222). The Emperor is able to evoke strong feelings of love and patriotism from the soldiers, and for the time being at least he has cured Rostóv of his cowardice. He even requests to serve at the front in the upcoming Battle of Austerlitz. It is interesting that love of Emperor is stronger by far than love of country or family for Rostóv and I think it is because the Emperor is a personification of the strength and valor of Russia. I am curious whether this new found devotion will continue to lead to feelings of bravery or whether Rostóv will let down the Emperor he loves.