Will talks about 'the steel' in Sal. How does Grenville make this aspect of her personality so vivid?

Read this extract, and then answer the question that follows it:
The shadow slid up the golden cliffs opposite and turned them to lead. As
darkness fell, the distorted trees went on holding the fraction of light in the
air.
The Thornhills squatted around the fire listening to the night, feeling its
weight at their backs. Beyond the circle of light, the darkness was full of
secretive noises, ticks and creaks, sudden rustlings and snappings, an
insistent tweeting. Shafts of cold air like the draught from a window stirred
the trees. From the river the frogs popped and ponked.
As the night deepened they hunched closer around the fire, feeding it
so that as soon as it began to die it flamed up again and filled the clearing
with jerky light. Willie and Dick heaped on armful after armful until the light
danced against the underside of the trees. Bub squatted close up to one
side, pushing in twigs that flared brilliantly.
They were warm, at least on one side, and the fire made them the
centre of a small warm world. But it made them helpless creatures too. The
blackness beyond the reach of the flames was as absolute as blindness.
The trees grew huge, hanging over them as if they had pulled up their
roots and crept closer. Their shaggy silhouettes leaned down over the firelit clearing.
The gun lay close to Thornhill’s hand. By the last of the daylight, out of
sight of Sal, he had loaded it. He had checked the flint, had the powderhorn
in his coat pocket.
He had thought that having a gun would make him feel safe. Why did it
not?
The damper was burned from being cooked too fast, but the steamy
fragrance under the charred crust was a comfort. The small noises they
made with their food seemed loud in the night. Thornhill could hear his tea
travel down his gullet, and the exclamations of his belly as it came to grips
with the damper.
He looked up at where even the light of the fire could not dim the stars.
He looked for the Southern Cross, which he had learned to steer by, but as
it often did it was playing hide-and-seek.
Might be they watching us, Willie said. Waiting, like. There was the start
of panic in his voice. Shut your trap, Willie, we ain’t got nothing to worry
about, Thornhill said.
In the tent he felt Sal squeezed up against him under the blanket. He
had heated a stone in the fire and wrapped it up in his coat to warm her
feet, but she was shivering. She was panting as quick as an animal. He
held her tight, feeling the cold at his back, until at last her breathing slowed
in sleep.
A wind had arisen out of the night. He could hear it up on the ridges,
although down in the valley everything was still. It was like the sound of
surf breaking on the shore, the way it swelled and then travelled around
the ridges, its whisper growing and then fading away. The valley was
dwarfed by the ocean of leaves and wind.
To be stretched out to sleep on his own earth, feeling his body lie along
ground that was his—he felt he had been hurrying all his life, and had at last come to a place where he could stop. He could smell the rich damp air coming in the tent-flap. He could feel the shape of the ground through his
back. My own, he kept saying to himself. My place. Thornhill’s place.
But the wind in the leaves up on the ridge was saying something else
entirely.
[from Part 3]