SAT Local Controller described a go around event when traffic awaiting departure taxied onto the runway without clearance; the reporter suspecting the pilot was unfamiliar with the 'Line up and wait' phraseology.

Narrative

The weather was Marginal VFR. A C172 was holding short of Runway 12R at Kilo Intersection. The C172 was given 'Runway 12R at Kilo; Line up and wait'. I scanned my runway to make sure it was clear. When it was; I looked back down the runway; but the C172 was still holding short of Runway 12R at Kilo. I instructed the C172 to 'hold short of Runway 12R at Kilo'; which he properly acknowledged. I again scanned my runway; and when I looked back at the approach end of the runway; the C172 was taxiing across the hold short line; I immediately transmitted to the C172 to 'STOP'. I instructed the C172 to make a '180 and exit the runway. The C172 exited the runway and was instructed to contact ground control on 121.2. A few minutes later; after the conversation with the FLM; the C172 was holding short of Runway 12R at Kilo and called ready for departure. I instructed the C172 to 'hold short of Runway 12R at Kilo'; which he corrected acknowledged. An MD88 had been cleared to land on Runway 12R and was 2-3 mile final. Again after scanning the runway to ensuring the previous arrival had cleared; I again scanned back to the approach end of the runway. The C172 had taxied onto the runway and was holding in position on Runway 12R at Kilo. I canceled the MD88's landing clearance and issued missed approach climb out instructions; which he appropriately acknowledged. The C172 was instructed to exit the runway and contact Ground Control. When the C172 exited at Kilo; he was nose to nose with a King Air awaiting departure. The Ground Controller asked for permission to utilized Runway 12R to Juliet intersection in order to get the C172 off of the runway. A LJ60 was behind the MD88 on final. I asked the LJ60 if he could land on Runway 12L and he replied 'affirmative'. I cleared the LJ60 to land on Runway 12L. Recommendation; make sure pilots understand 'Line up and wait procedures'.