Register can be a tricky concept for a lot of students. They often focus solely on the grammatical plane and forget about the social context in which it occurs (most EFL textbooks don’t really help in this regard). On the other hand, one mistake I think that some students (and teachers) make is thinking that register is determined solely by mode and that the situation automatically dictates language choice – that business English = more polite. If you look at the business e-mails section on the BNC, for example, you’ll find they are mostly about football. Important in register is also field and tenor choices.
I think, however, it is not the context of situation itself that is the sole determiner but the participants’ perception of that situation that leads to variation in language. Register is not a pre-existing independent set of truth-conditions but is negotiated by the participants as the situation unfolds. There is a wonderful Peanuts cartoon that illustrates this quite well (you can see it here) where Snoopy is sitting outside and one by one other characters walk past and greet him. The interesting thing is that, while the mode and field is the same, each character varies slightly in the level of formality used in the greeting, from Lucy’s simple ‘Hi Snoopy’ to Violet formally shaking Snoopy’s hand, reflecting perhaps differences in how each character perceives the tenor relationship between themselves and Snoopy. It’s quite a useful little cartoon to use in class.
Register is not, however, open-ended choice but is, I think, also constrained by the context of culture. I think the relation between the two is that the culture makes certain linguistic choices available within a given context of situation from which the participants are able to draw from as the text unfolds. This is one reason why the Peanuts cartoon is funny, in that it plays with our expectations of what is permissible within that context. I think this is also an important point for EFL teaching in general as well. It is often the teacher, or the textbook, who determines the linguistic resources for the student to then use, and the student is judged on how well they use these predetermined lexicogrammatical choices. I think instead the role of the teacher is to provide the range of lexicogrammatical options available from which the student may then choose. I had an Advanced student once who, no matter how many times we would ‘practise’ polite requests, would invariably end each lesson with a direct “Please give me that paper”. But then I realised that he was just that kind of guy.