Interesting observing a lesson today which opened with fairly open, personalised conversation and then moved onto the coursebook material for which the opening conversation had been preparing them (thematically-speaking).
Two things happened: the energy level palpably dropped, not so the class became completely flat but so the buzz which had been there disappeared; and the students simply stopped producing much language. My impression was that although it looked as if something was happening, nothing much actually was. The sentence-level contributions they had been making, whether prompted or not, dwindled to silence or mono-syllabic attempts to negotiate answers to two sets of questions on the same listening exercise.
In place of conversation about participants' own families was a series of 'exercises' concerning a (real? made-up? who cares?) family. This is what we mean by materials getting between the teacher and the students - not in a way that damages their relationship, but in a way that renders the interaction so bland and removed from real concerns as to be almost redundant.
It also occurred to me why monitoring pairwork is so unsatisfactory - you just don't hear enough of each conversation, and when you approach students frequently stop or alter what they're saying. This may be another argument for prioritising whole-group work.
In short, whatever language learning took place - and we can assume from the relaxed atmosphere that the conditions were right for learning - did so not because, but in spite, of the coursebook material.