Welcome to the forums…and congratulations on your EY interview!
If you haven’t found it already, you are likely to find the EY wikis of use regarding company specific interview information. See, here - Ernst & Young Interview Questions | WikiJob - and, here - http://www.wikijob.co.uk/wiki/ernst-young.
Regarding competencies, the STAR technique is ok. Personally, I think answering that way is slightly overly structured and unnatural, but if you do want to simplify things and work to a structure it may suit you. We have general information on competency interviews, here - http://www.wikijob.co.uk/wiki/competency-based-interview - and more specific information on competency based questions (which sounds like what you’re looking for), here - http://www.wikijob.co.uk/wiki/competency-based-questions.
For the “what differentiates EY from its competitors” question, think about what impresses you personally about EY, compared to other similar firms. Interviewers are trying to find out if you have fully researched what they do and how they do it and they are notably also trying to find out if you are actually interested in working for them specifically, or if you just applied because they are a big firm and you want a job in the industry.
For this reason it’s important to tell them what impresses you, personally about EY and why you have chosen to apply to them, and why you want to work for them, more than their competitors. Actually think about why you want a job at this company, think about criteria you look for in a company and how EY fulfil these criteria (in terms of specialisms, international reach, history, working style, social life, the compassionate work they do around the world, etc, etc, etc). This question is actually very useful, as researching your answer should help you to figure out if this is actually a company you do like and want to work for, or if actually, you wouldn’t want to work here in a million years!
Hope this helps… very interested to find out how things go for you, please do let us know. Any more questions just post them right back up here