Why does Steve avoid talking to King in the courtroom?

Steve avoids talking to King because the association could make him seem guilty in the eyes of the spectators in the courtroom.
When James King and Richard Evans attempt to rob a drugstore and kill a clerk, Steven Harmon is implicated because he heard them plan the crime. His lawyer knows that any perceived association between him, King, and Evans will make him look guilty and reduce his chances of being cleared of the crime. He advises his client to not speak to King when they're all in the same room.
When Ms. O'Brien, his lawyer, says that the jury may not be able to see the difference between him and the "bad guys" taking the stand, it's clear that they'll have to differentiate Steve from them. Their background and upbringing already make them associates in the eyes of the people in the courtroom. Only by not speaking to him can Steve separate himself from the two men involved in the robbery and murder.

James King is portrayed as a heartless criminal, who plans the robbery and initially asks Steve Harmon if he is willing to join the crew. Throughout the trial, O'Brien explains to Steve that he must distance himself from James King, who is more than likely going to be charged with murdering Alguinaldo Nesbitt. In an attempt to distance himself from James King, Steve avoids speaking to him in the courtroom to prove that he is only an acquaintance of James and not his close friend. There are also several scenes where James attempts to speak with Steve before they enter the courtroom. In one scene, James attempts to intimidate Steve by staring at him, which Steve finds amusing. Steve also avoids speaking to James because he is too focused on his own future and situation. Overall, Steve regrets having anything to do with James and avoids speaking to him during the trial in order to create distance from the criminal.