As far as preparing yourself for an interview for an ICU, convey the experience of taking care of critically-ill patients. When they ask what your strengths are, this is a good chance to demonstrate your knowledge of PA caths, vents, etc. Most ICU directors are looking for employees who will do everything possible to ensure patient safety and make sure proper care is given to the critically-ill. Another thing is to be sure that you know your limits. Managers definitely don't want employees who are usually pretty autonomus, to not ask for help when it's needed.
For ER it is important to demonstrate qualities of recognizing acute problems and knowing what changes are important to inform the MD of. In ER, usually the patients are highly likely to have fast changes in their status. Remembering that most of the time patients that come to the ER are most unstable at this time is crucial to being an ER nurse. Also make sure that you show you are a team player and that you are fast at what you do but in a careful way. (things move quickly in most ERs)
Good luck with you interviews and try for the ICU position, you may decide to return to school one day.