The tone of voice, the level of language, the body language used, and even jokes that are added all effect the strength of the speech. If a speech is boring or odd then it will not be effective. Memory is a canon that is not as important as it once was. Today politicians that give speeches use electronic messages or glance at a written speech before them. Now even some arguments are fought out over the computer without speech ever occurring.
Delivery is the most important canon in speech communication.
The pisteis are another way of looking at how effective a speech will be. The ethos of a rhetor that is speaking is not more important than the ethos of a rhetor who writes texts. However, pathos is extremely important in the effectiveness of a speech. As written above, style plays a large role in the pathos of a speech. If the audience does not like the style of the rhetor, he will have a tougher time convincing the audience.