I believe that the heroes and the Gods have an influence on the plot of the Iliad. For example, in book 18 Hector does not take the omen from Zeus to go back to the camp. In book 1 though, Zeus and Apollo influence the plot where Apollo causes the plague, and Zeus becomes in the favor of the Trojans because of Achilles' mishap.
An arguement can be made for both, of course, but I believe that this is mostly driven by the character, not the gods. This is because ultimately the direction of the plot flows from the heroes' decisions to act and to choose, despite what the gods say or do. The gods interfere, but in the end, it is the hero's journey.