It became interesting to you because the teacher was so enthusiastic about it that you couldn't help but become interested. All of that may have seemed quite spontaneous to you as the student. The teacher was very, very cool. The teacher was also very, very organized. This kind of teaching is an art form.
Some teachers have a real knack for it.
The class that seems to be very loose and open-ended is actually keeping right up to date with the required course study mandated by the school. At the end of the semester, the entire course outline has been delivered right on time. There WAS a plan all along and the teacher stuck to it-by being organized. Here is a worksheet for developing a lesson plan or course study. You can make up your own format, but this sequence seems to work very well.