Possible Clues:

• Type of body language considered rude in many cultures
• Visual communication?
• Fixed gaze from detective not favouring discretion
• This should keep viewers in touch
• Big wheel in London to call meeting of viewers
• It's capable of bringing the viewers together to spy on informer
• Recording of iPhone perhaps that's shared between two viewers
• Asset in oratory
• Asset for a public speaker ... or a hint to 17-, 24-, 37- and 52-Across
• Aid for establishing rapport