A colleague recently asked me whether I had any opinion on what an invited speaker at a conference should do to repay the confidence the PC chairs and the conference organizers placed in her/him. I do not have much experience on this matter myself, but, for what they are worth, here are my opinions, after having had a peek at the Platonic world of ideas under the voice "ideal invited speaker". (Caveat: I realize that what I write below is an ideal that is hard to reach in real life since we are all pressed for time and we all have a lot on our plates. However, as Salvador Dali said, one should have no fear of perfection --- one will never reach it.)
Let's start with the most obvious role played by an invited speaker at a conference: an invited speaker should deliver a well polished and clear keynote address that is broadly accessible to the conference attendees. Fulfilling this goal requires a careful choice of the topic for the invited talk and preparation, as well as a rough idea of the profile of the typical conference attendees. Of course, some of the participants in the conference might not appreciate all the details of the material covered in the talk and some of it might be addressed to experts, but the talk should have a clear message and should give everyone in the audience something to take home. (I believe that this applies to every talk one gives, as Gian-Carlo Rota said in the first of his ten lessons he wished he had been taught.)
The delivery of a good talk is, however, by no means the only contribution that an invited speaker should give to a conference. In my book, an invited speaker should take active part in the conference. This means that (s)he should attend most, if not all, of the conference sessions (assuming the conference does not have parallel sessions, of course), ask questions as appropriate (and in particular, if no question is forthcoming from the rest of the audience), discuss with the conference participants during the breaks, mingle and take part in the social programme, and serve as an example for the young researchers in the audience.
This is hard work, but being an invited speaker is a great honour that comes with some responsibilities towards the PC chairs, the organizers and the community as a whole.