The way I saw it was the Fallen was an interesting villian. I viewed him as lazy and not really wanting to do anything and had the Decepticons do his work for him. He was just lying around the decepticon lair spouting orders and saying that he can't do anything until the last Prime is offline.
Once Optimus Prime went offline at Megatron's hand he finally got up from his "Bed" but it seemed more like "Man, now I gotta get up and do something, I just lost my last excuse".
I thought is was subtle and done well. Even when the Fallen was on Earth he did not do anything until he had no choice or he would look bad to his underlings.