Remember to remain mysterious. Talk less but make your words powerful and meaningful. Effy's lines always have an underlying meaning to them.
Also keep in that mind that Effy has some emotional issues that trigger her quiet yet erratic behavior. Her depression shows later on in the show, but know that she is in fact depressed. She doesn't really care about anything and what happens. If you're the type of person to disassociate and say "f--- it", then you can be the perfect Effy. You simply do not care, and are here for fun.