My own personal usage, once I stopped to think about it, goes like this:
Italics are what I use most often for emphasis, usually if something is said with sarcasm or an overly dramatic tone. I also use italics to indicate a foreign language (Madre de dios), or, less often, someone speaking a foreign language but it being transcribed in English (Mother of God, he muttered, in Spanish). If someone is having a stream of consciousness dialogue in their head (talking to themselves), I tend to use italics as well, or if they are quoting someone or something. I use it for onomatopoeia (ribbit, ribbit) and for personification (drip, drip, said the tap.)
Bold tends to indicate someone being surprised or shocked. (Jesus Christ!, You have got to be kidding me!). I also use it when someone is angry (That is not my fault!) or being self-deprecating. Less often, I use it for sound effects.
So, if you care to weigh in, how do you use your emphasis devices, mes amis?