Suddenly I heard a step near me; quiet automatically I sprang back from the wall and began walking on with, I fervently hoped, an air of rumination and unconcern.

Explanation

The error is in the adjective "quiet", which should be replaced with "quietly" to indicate the manner in which the action was performed.The correct sentence would be: "Suddenly I heard a step near me; automatically I sprang back from the wall and began walking on quietly, with, I fervently hoped, an air of rumination and unconcern."