Accommodation is the modification of your speech style in order to attend to what you believe to be your listener's interpretative competence and conversational needs, and to your respective role relationships and task-specific aims. These beliefs may be modified during the course of the interaction depending on your experience of your listener's actual competence. You may also change your speech style in response to the way your listener talks:
● if A asks B a question, B is likely to answer
● if A talks a lot, B is likely to talk less
● if A pauses, B is likely to start speaking; if A summarises B’s point of view, B is likely (initially at least) to agree
● if A laughs, B is likely to laugh also
● if A speaks in more or less 'Standard English', B is also likely to try to do so
In other words, there may be social/psychological explanations for accommodation (beliefs) as well as interactive ones.
You may have assumed before now that ‘meaning’ is something that is ‘delivered’ by speakers to hearers, without any effort on the hearer’s part. But all the hearer gets is a stream of sound: vibrations of molecules in the air which impinge on your eardrum. It is the hearer’s job to decide what these vibrations mean and one of the ways they can do this is by checking with the speaker; this might be by repeating what they hear, asking a question, summarising, creating a silence (in order to elicit a repetition or rephrasing) and so on. Often meanings only become clear through joint action. In this sense, meaning is negotiated rather than delivered.
Above we listed a wide range of possible macro-contextual factors that explain variation in how English is used, even by native speakers (e.g. region and class). To complicate matters further, there are also micro-contextual factors that emerge as conversations progress and speakers/listeners react to patterns in each other's talk. All in all, this adds up to a lot of variation!