As Herself: Katy Perry is supposed to appear in mid-2013, to quote from British showbiz and football forums - if this is confirmed.
Rachelle Lynne and Kate Webb were named in one episode, their name-tags showing clearly to the camera. However, this is something of a subversion - they are not Adam Westing, but not playing themselves either, the reason being unknown.
An Aesop: Plays the trope straight, particularly from 2007 onwards.
Possibly, this also encouraged Scrubs to create an expy of Nikki, albeit non-speaking.
Fanservice: Two extras: one tan-skinned and the actress playing her is currently unknown, and a blonde extra whose actress is currently unknown, but she gets referred to as "Nikki" on-screen a few times. Another one, called Kate, also as well, is considered Fanservice. The Fandom for them, is well, huge... making them into an Ensemble Dark Horse.
However, they are not there for that purpose, but as functional ( if non-speaking or rarely speaking) characters. Word Of God on this has yet to happen.
Insufferable Genius/Rightly Self Righteous: Just about the entire cast. The entire medical team are ripe with attitude problems and constantly go over the heads of officials and regulations, it almost always only helps in saving patients lives.
Karma Houdini: Subverted with Ric Griffin - He stole Michael Spence's wife, and generally comes off as smug when he constantly relays messages to Michael from his ex-wife & Michael suffers a setback in his career. It's a subversion, because Ric has cancer, which appears to be both Laser-Guided Karma & letting him get away with all of this as other characters feel sorry for him. At the same time.
Played straight, however, with Oliver Valentine, who looks as though he's going to get away with continuing to practise as a doctor illegally.
Prodigal Hero: Henrik Hanssen, who left Sweden after his father was disgraced for using Nazi medical research. Henrik considered this unforgivable since his mother was a holocaust survivor and blamed his father for driving her to commit suicide. Turns out, his mother convinced his father to use the data and she killed herself for an entirely different reason.
The Red Stapler: Jac Naylor's outfits, which have become suddenly fashionable.
Plus the unnamed tan-skinned nurse's civilian wear too...
Those Three Girls: Nikki, Kate (two blonde girls) and a tan-skinned Ambiguously Brown woman (not sure if she's half-Indian, Portuguese/Hispanic or some other race - picture on the left here◊, who looks in her mid-30's. They always appear on the nurses' station in most scenes.