It depends on what kind of engineering you are doing?
If you are a mechanical engineer then your study background will be such that you can converse and answer questions about mechanical engineering. However, this stuff is less important becuse they will know that you have studied mechanical engineering (if that's your background) and you should be proficient with the subject.
They would propbably be more interested and focus on your approach to problem solving and your awareness of cost, quality, function, time, environment and risk when running a project. they will also want to see that you won't always go for the most expensive solution if it is not necessary, and conversely that you won't go for a cheap solution which might compromise product funstion (this is a very basic example).
Engineering requires a constant second anture ability to be able to weigh up the pros and cons and impacts of decisions. You never do things just for the sake of it, you need to be able to demonstarte logical reasoning and evaluation of problems.
During every engineering interview i have attended, i have been asked to describe how i have solved a complex problem and how i have managed it through to completion. When you describe how you solve a problem, they will normally want to see that you make desicions based upon sound evidence logical intuition, so that they can be shore that you won't waste their time and money. Project planning is also an area they will address.
But to be honest interviews vary, but like i said it willl depend on the type of engineering job it is?