From the very beginning of the Iliad, when the poet asks the Muse to reveal how "the will of Zeus was accomplished," we know that the events we are witnessing have Fate's fingerprints all over them. Time and again, we are reminded how it is impossible to escape one's fate; to some characters, this thought is comforting. That said, just because everything is fated doesn't mean there isn't any freedom. Achilleus, for example, has a double fate: if he goes home, he will live long without glory. If he stays at Troy, he will have lots of glory, but a short life. So he has a choice. Also, it is important to recognize that the gods don't control fate; there are times when they consider acting against it. Usually, though, they think it's best to do what fate says, just to make sure things don't get out of hand.
Even though Achilleus is able to choose his life's path, he still does not have complete freedom of will.
Gods and mortals are equally free to disobey fate. The difference is that gods know what it is they're disobeying.