I've done a bit of searching for this phrase and found the following:
Idioms & Phrases
Involve oneself or intervene, as in He knew he'd be able to step into a job in his father's firm , or Jane asked Mary to step into the matter and settle it . Also see step in.
However, the place I first heard the phrase is on Law & Order: Criminal Intent, where Deakins, the supervisor of the Major Case Squad, uses it with a direct object of a suspect or a witness, which barely fits the definition. The definition implies usage towards a situation or position.
Exchanges on the show usually go something like this:
Detective: "We don't have any real leverage on him to find out what he knows!"
Supervisor: "Step into him."
Detective: "As long as his lawyer is protecting him, we'll never get him to talk."
Supervisor: "Step into him. Hard."
Detective: Re-enters the interrogation room and squeezes the information from the suspect over the lawyers increasing complaints.
That seems to imply there is another, even more idiomatic usage for the phrase with a meaning closer to "investigate" or "harass". How common is this idiom? Is the screen writer mis-using a legitimate idiom? Or is there another meaning or police/detective slang that's not so documented around the web?