A face dancer was a member of a race bred by the Bene Tleilax. Their standing in Tleilaxu society fell below that of Tleilaxu Master, and their ability to mimic other individuals made them popular for numerous roles, including spies and assasins.
Tleilaxu Face Dancers had the ability to take on the physical appearance of other individuals. Their name came from the fact that when they changed shape their faces would appear to 'dance' as the flesh moved.
Despite Face Dancers being neither male nor female, they had the ability to take on the physical sex of the individual they were copying. However, they were incapable of breeding.
Face Dancers could generally mimic individuals to an almost perfect level of precision. As a result, the Bene Tleilax would often use them to infiltrate other organisations and governments. Ideally, the Bene Tleilax could replace almost whomever they wished with a Face Dancer, and would often do so, with some of these impostors acting successfully for many years.
However, the use of Face Dancers was not an infallible maneuvre. There were two significant limitations.
The first limitation was that their pheromones could be detected by a Bene Gesserit adept. A Bene Gesserit Sister or Reverend Mother could detect them by a subtle but distinct odour emitted by their pheromones. Indeed, the Bene Tleilax generally were not aware that the Bene Gesserit had this ability and used it to their advantage. As a result, Face Dancers were not as effective as the Tleilaxu had hoped, when the Bene Gesserit were involved.
By the time of the arrival of the Honored Matres, the Tleilaxu had developed Face Dancer talents so perfectly, that even the Bene Gesserit had difficulty detecting them. These new Face Dancers were also capable (through a touching of foreheads) of acquiring some of the memories of those who had just died, so that when they were copying those dead individuals, they were also able to discover some of their secrets, and play their role as impostor even more convincingly.
The second limitation in the use of Face Dancers was that at times, if a Face Dancer had been acting in a certain role for a long time, the Face Dancer would become delusional and start to believe that they were in fact the person they were mimmicking. When this happened, they would fall outside Bene Tleilax control and would need to be killed, often exposing the Tleilaxu infiltration in the process. This happened with the Rakian Chief Priest Hasmar Tuek.