Lots of beginning dialoguers have trouble with improvising a subject. It's something having to do with surrending how they are going to spend their time with no plan, no objective, no job, equal authority, and mostly people have never improvised a conversation in a group situation. The problem with using metaphors to inspire is that many people say they understand the example, but then it turns out they are not able to demonstrate the skills necessary to actually do what it is they say they understand.
If you like metaphors, how about this one: Imagine a group of people are teaching themselves conversationally how to speak a language. To do this, they would usually practice scripts to establish context about what are the appropriate situations to infer and interpret meaning from. So they establish so they know, not just what is being said, but the context of where and when words are happening.
In Dialogue, the group can create from scratch their own context of how meaning is assigned. To do this, they explore the meaning that comes up, so it happens gradually that meaning is assigned gradually as the similar light of recognition comes on in every participant. Those who come late to the group can't quite tell what is going on, but it just looks like something different is happening. Most of us would like the Dialogue activities to be transparent enough that someone would be able to participate if they can be observant enough to see what is happening, ie: for the Dialogue to be in English and not have to know special secrets to decipher it.
The group does this by being aware of "frames" of meaning, where the content changes in terms of what is beneath, above, aside or associated with the subject(s). So as everyone says something about what they think the subject is, the thread of meaning weaving through becomes obvious to some, is hidden from others, and goes off on tangents that never come back around for some. In the end, you'll have an experience of holding so many different points of view at once that it will spin your brain, because they all arrived at a different time.