The Smilers were mounted in booths around the Starship UK. They served primarily as the security for the spacecraft, but also performed other services such as information consulting and automatic teachers who enforced strict grading systems for pupils. When adults broke the law or were deemed unnecessary, the Smilers would feed them to the star whale; children were sentenced to work in the Tower of London. The Smilers rotated their heads to show three faces: happy, displeased (for warnings) and livid (when attacking).
Smilers normally displayed a comforting happy face if citizens were obedient, though by their very nature as the tenacious watchdogs of the starship, their smiles were more often than not unsettling, because Smilers would switch to grimacing expressions if they witnessed anyone disrupting the order on Starship UK. This was the one and only warning they gave for the offender to immediately desist from their actions. If this warning was ignored, Smilers would reveal enraged faces with menacing red eyes and administer a brutal punishment.
While Smilers appeared to be merely heads, in extreme circumstances their entire bodies would leave their booths and attack. The terrified population never dared approach the booths, which meant the area surrounding them was completely clean, as the Eleventh Doctor noticed. The Smilers quickly recovered from damage such as being shot through built-in repair capabilities.
In an alternate dimension in which the Doctor's adventures existed as a fictional TV show called Doctor Who, a fan dressed as a Smiler was present at a Doctor Who convention which the Eleventh Doctor visited with Ally in 2013. (COMIC: The Girl Who Loved Doctor Who)