You'll have been sent a handy little guide from Northern once they confirm your interview date and time. Essentially it'll tell you that the questions will be competency based.
"Tell me about a time you have followed rules and procedures?"
Or "tell me about a time you have worked in a team?" Those sort of questions you'd kind of expect to be asked.
Theres no need to rush your answers. Use the old sip a glass of water trick to get some thinking time. And if you don't understand the question, be honest with the interviewers, they may be able to reword it for you. There was a question I wasn't sure of and asked them if they could reword it.
It's best to answer these questions using the STAR technique;
Situation
Task
Action
Result
Again all of this will be mentioned in the guide sent out. Don't be alarmed if you haven't received this guide from Northern, it took a couple of days for them to confirm my interview time.
Also do your own research on the company just incase you get asked what do you know about Northern.