Characters
Don’t start tryin’ to do the right thing, boy-o. You haven’t the practice.
— Captain Dudley Smith · L.A. Confidential

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker scorns the listener's sudden attempt at decency, calling them 'boy-o' and saying they lack the practice. It is a condescending jab: goodness is treated as a skill, and this person's record shows they haven't earned any fluency in it.
Context reviewed