This isn't going to be a statement of a formula, but a series of reflections about teaching TOK, based on my own personal experience and on discussions with colleagues.
When I began teaching TOK twelve years ago, I used to favour the lecture and random reflection mode, where I would introduce a topic with my own ideas about it, presenting my own ways of organising the different concepts and their relationships. I would then pose a few questions for discussion by the class, either on the basis of a handout, or an activity.
I noticed quite quickly that this didn't work very successfully, because the lecture turned into passive listening by those who chose to keep their ears and minds opening, into a slow warm bath of words in which my students would sink comfortably while they fantasized or doodled. Occasionally someone would ask a question to break the monotony. The discussion became a private chat between a handful of interested students, requiring nothing from the others beyond the courtesy of keeping quiet, but I discovered later in another country that even this courtesy couldn't be taken for granted. I had seen some wonderful discussions conducted by two of my colleagues where they seemed to sustain the interest of their students, but I was deceived by how easy it seemed, and thought mistakenly that I could imitate them.
Over the years, I realized that one couldn't just lecture to students the way they did at universities - as if to an interested audience. One can't take the interest of students for granted, but has to stimulate it. In other words, students have to "prepped"for a lecture as much as for anything else, and the lecture itself needs to have what are in the jargon called "advance organisers". Also, the lecture must be stimulating at least in the sense of challenging what students take for granted, and humorous (although this can be dangerously risky). It must also have a clear structure and progression, and contain recaps and summaries at key points.
The discussions, too, need to be focused. Asking the right questions at the right time is an art, a major part of which required one to be comfortable with silence. A good technique, I discovered is to write down key points as they develop, so that the discussion could be tracked visually. This conveys the message that students too are capable of making points worthy of being written down (especially in a pedagogical culture that discourages student discussion).
My worst lessons are those that turn into monologues on my part, or private and unstructured chats with some students. This almost always happens when I go into class with little or no preparation or forethought. I say "almost always" because on some rare occasions, I was able to engage the class in a very good discussion, usually by means of a fortuitious conjunction of circumstances, even though when I entered the room, I had very little idea of what to say. On the other hand, some of my best prepared lessons have occasionally gone spectacularly wrong.
I am beginning to get around to the view that TOK ought to be based mainly on some activity - not just reading and responding to some pre-assigned text (which many students in any case seem loath to do). The activity ideally involves some manipulation of objects and reflection on the process, and includes some discussion among students with a clearly designed process and purpose. However, the occasional lecture would still be needed to set the framework for the activity.
On this hypothetical belief rests my case for the semester retreat for mostly lectures with some discussion and activity, with the classtime throughout the senester being used for other activities, student presentations, responding to journals, and essay writing advice.