Remember that it's very difficult for some customers to approach someone in authority to discuss the behavior of another customer.  Some people are afraid of reprisal, some don't want to get involved, and others are simply timid.  Before we start the interview process, it's a good idea think about how we want to communicate with the person lodging the complaint.  Therefore, 

• Be attentive to the customer's concerns.
• Demonstrate by your body language and verbal responses that you take the person's concerns seriously.
• Demonstrate a sincere desire to be of assistance.
• Listen carefully.
• If you think it might help the complaining customer feel more comfortable, ask if he would like to move away from the public area to discuss his concerns.

With those basic guidelines in mind, you can follow up with questions like these:

"I'm sorry you saw something that was offensive (or upsetting... mirror the patron's complaint as much as possible.  Let the patron know you heard his complaint.)

"So that I can deal appropriately with the situation, please tell me what happened."  (Try not to "lead" the complainer.)

Depending on what the complaining customer says, you can follow up in several ways.

• "What drew your attention to the screen?"
• "What exactly did you see on the screen?"
• "Did the other person try to get you to look at his screen?"
• "Did the other person attempt to engage you in conversation about what he was viewing?  Did he speak to you at all?"