Your interviewers will expect you to be able to do "basic" things without thinking (such as sketch $y=ln x$ for example), however they will understand that you might have covered different things.

A good way of practising for interview is to try solving questions in front of people (if there are a group of you applying you could take it in turns). It doesn't really matter what the question is as long as you cannot do it immediately! STEP questions are a good source of practice questions - again it doesn't matter which paper or topic as long as you find it hard!

In interview they are going to want to see how you tackle difficult problems, and whatever question they ask they will extend it until you find it difficult! It is important to keep talking as you are trying to solve the problem (you will get pen and paper), and also to listen to what the interviewers are saying as they might give you snippets that you can use to tackle the problem.

It is a rather old video now, but here is an example of what an interview might look like. Note that the actual interview would have been about 30 mins, and the questions did get considerably harder which is not shown (IIRC the first one ended up with series expansions and approximations). The important thing to note is that the candidate keeps talking whilst solving the problems and reacts to the interviewers input.