Usually, it's two people interviewing, as it allows one person to ask questions whilst the other takes notes.
It also allows a discussion after the interview rather than just one person's opinion.
The purpose isn't to trip people up, but to find out your thought processes and how you react when being stretched.
It isn't just about technical ability, but also about how you behave, think and work with others - whether colleagues or customers. It's far easier to teach somebody practical skills than it is to change them from a cocky arrogant ass into a courteous representative of the company.
Do you think things through before acting (or opening your mouth)?
Why do do take certain actions? What's the reasoning for doing (or saying) that and not something else?
Do you fall apart when things go wrong? Or do you pause, think and plan?
Most stuff is logical and you probably do it all the time in daily life without realising, so it's just breaking it down into small bites to explain the reasoning. There isn't necessarily a 'right' or 'wrong' answer to many of the questions, but more about the rationale for making that decision. You can make a wrong decision, but for the right reasons!
It's then about showing you can learn from things that went wrong, so they don't happen again.
Simply, if you were the customer, how would you want or expect to be treated?
Research
SOARA for typical stucture.