Waitrose interview questions

Very long interview, was there for a part-time role and ended up being there around an hour and a half. The interview was with the store manager, spent the first half just chatting about anything and then the second half was the actual interview. The manager flicked through a booklet and picked random questions to ask me. It seemed quite unprofessional, there was no structure to the interview. Thought I'd definitely got it because I was there so long, but I didn't. Felt like such a waste of time.