War. What is it good for?

The images that the audience is given early on in Othello help to create the ideas of the natures of the characters. It is an oft-quoted statement that everyone is at their most honest during a war for fear of imminent death. When a country is at war people are more forthright about their feelings and what they really think. After all, there is no way to be sure when someone will die, or whether or not they will ever again see the person to whom they are speaking. This can be seen in Act I when Othello is explaining how he and Desdemona came to be married. He told tales of war to her and wooed her with them. The danger drew her to him, and she acted on her feelings immediately, as one does in war.
The setting is also given, because Othello goes to Cyprus to fight the Turks in Act I. So the audience is immediately able to deduce that the characters are living in hard times. There are certain elements that remain the same throughout wars, such as death and pain. Tension is caused from this knowledge. In Act I, literal war is discussed between the Senators, Othello, and the Duke of Venice. That helps to lay the scene.
Characters are not completely explained by their war-time actions, because at the very beginning, there are only stories of war. However, Othello is heralded, and praised for his bravery and competence in battle; he is therefore a confident and competent man. Cassio is untried in war, which makes him not necessarily an innocent, but at least not as hardened to life as the men who have fought.