Ethos is one of the three rhetorical, or persuasive, appeals that Aristotle, the Greek philosopher, discussed. Ethos is an appeal to authority or credibility. The other appeals are pathos, an appeal to emotions, and logos, an appeal to reason.
Watching or even listening to Martin Luther King, Jr.'s, "I Have a Dream" speech has a more powerful ethos appeal than simply reading it. Martin Luther King, Jr., delivered the speech in Washington, D.C., in front of the Lincoln Memorial to a group of 250,000 people who had assembled as part of the largest gathering that had ever met in the nation's capital. Just seeing the crowds is impressive, and that helps build MLK's ethos. In addition, when you listen to or watch the speech, you will hear audience reactions punctuating certain phrases. The approbation of the crowd makes you want to respond in the same way, again bolstering the credibility of the speech.
Some of the specific appeals to authority MLK uses are reinforced by the setting, which comes across better when you watch the speech. MLK refers to the Emancipation Proclamation, signed by Abraham Lincoln, before whose statue he stands. And as he draws on the authority of the nation's founding document, the Declaration of Independence, it is all the more stirring when you see him standing in the seat of the very government that eventually rose from that document. Finally, when MLK calls upon the authority of the Bible or Negro spirituals, his claims are made more powerful when you hear his voice because the cadence and dialect he uses reflect the tone of a Southern gospel preacher.
For these reasons, to appreciate the full power of the ethos appeal of this speech, you should watch it rather than just read it.