This passage is from The Children's Bach by Helen Garner:
She turned round with her arms full of stinking sheets. He had not heard a word. His eyes had gone out of focus, his pitch was up, his pace was accelerating, his smile was the one-sided, manic grimace of the born raver: he was away on the high seas of narrative. In the wide planting of his feet, his blithe assumption of an audience, she saw Dexter, oh poor Dexter, gone away on a plane to try and pull the sword out of the stone. Her insides quivered with what she thought was laughter. She pushed the chattering boy aside: he turned to follow her, pointed his rapt, jabbering face in her direction, but she stepped out into the passage and closed the door on him. He took no notice: as she walked away his chipmunk voice rattled on without interruption.
Does it mean: she saw him in the place where there were the big footprints of Dexter and his footprints reflected his calmness to someone who was seeing his footprints?
Dose planting of his feet mean his footprints?