The looks of shock, disdain, horror etc. on the nurses' faces sometimes. Mainly happens to characters Kath (and if I'm correct) Rhona when a patient comes in.
With regard to Kath, she's mentioned in the main Casualty article.
The expressions that Ruth has on her face can sometimes be Narm.
The episode that introduces Ben Chiltern - a guy who secretly took up residence in Robyn's loft, turned out to be a nurse and managed to get a job at the Hospital after she got her foot impaled on a rake. Now his nickname is 'Lofty'. It helps that he's fairly tall too.