Thinking of the Speaker
Answer: A public speaker makes a great effort to prepare and deliver his talk. Seeing someone more interested in talking to his neighbor than listening to him can be most disturbing. Not only will you be guilty of hurting his feelings, but also you may be compromising his effectiveness in delivering his message.
If you cannot resist the temptation to share your mid-lecture thoughts the best thing to do is to write a note that you can pass to your neighbor. The speaker will generously attribute your writing as an effort to record his words and will not miss a beat in his presentation.