I think this is very dependent on the firm, your interviewer, or even just how you personally are responding to the interviewer.
My first ever interview was just an easy conversation about school, uni life, the firm etc. About 30 min in the interviewer was like, oh I should probably ask you some of the set questions, and asked me like 2. So in that case, definitely not a standard set interview, but not random either.
At another one there were 2 interviews. The one with a partner was also entirely conversational,and he took very little notes. Then there was one with a manager which was very much just them shooting questions and taking notes on your answers.
My other 2 interviews were both 2 on 1 interviews, and were conversational, but explicitly guided by standard questions. In my experience, there weren't really any random questions or going very off track, and I haven't heard much about it happening so I don't think its that common.