Tower Local/CIC noted an unauthorized runway crossing missed by others in the Tower; the reporter listing a number of corrective actions that might prevent future occurrences.

Narrative

I was working Local Control/CIC position. I was looking at my runways when I noticed a general aviation aircraft taxiing across Runway XXL at Bravo. I interrupted the conversation going on between the Ground Controller and Flight Data and stated; 'which one of you is talking to that aircraft!'; and I pointed to the plane that was now in between the parallels. I attempted to reach out to the aircraft on my frequencies. The Ground Controller attempted to do the same. After reviewing the tapes (Ground Control); it was discovered that the pilot read back the correct runway assignment. I feel that the two controllers should have played a more vigilant role in LOOKING OUT the window. We had a similar event; same scenario happen earlier this month. I feel if there had been less socializing and more attention to aircraft this event could have possibly been prevented. Recommendations: Perhaps we need better signage at the intersection; it is in fact a hot spot at our airfield. Maybe a warning sign could be displayed at the intersection. Pilots need to pay attention to runway environment or ask controllers for assistance if they are uncertain. Less socializing; controllers need to do their job and maintain vigilance by looking out the window because situations can occur at any level or volume of traffic. Encourage higher standards of personal professionalism of controllers for the sake of safety.