She cast a fearfull glance at the storm clouds gathering on the horizon.

The fearfuller dog hid behind the couch whenever strangers came to the door.

She felt fearfuler than she had before entering the dark forest.

She fearfully approached the abandoned house, her flashlight trembling in her hands.

As the thunderstorm approached, the old dog let out the fearfullest whine, and everyone in the house fell silent.