I've only just noticed this difference but it seems to be pretty universal:
Here in the UK, a smile is a mild, pallid creature, a bit like a 'nice' person. It can be deeply felt, but is often either restrained or polite or even forced. A grin, on the other hand, although performed with closed lips, is genuine and expresses a deep sense of gleeful mirth! A smirk, at least to my generation, is a rather sly half-grin, which has connotations of derision with hidden or less well-concealed, spite towards another. It is usually employed by those with poor self-esteem who wish to see themselves as superior to the person they are thinking about. We would use the word 'grin' to describe a wry smile that was prompted by fondness.
In current US romantic fiction, at least, these conventions seem to be reversed. People start off with a grin, and then it expands into a heartfelt smile. A smirk does not seem to have the same level of negativity associated with it, either: the first time I read a heroine saying how much she loved that the guy always smirked at her silly ways, I thought, 'How low is her self-image, to enjoy being looked down on and almost bullied like that?' It turns out that the problem was one of translation between English and American(!!)
Free Proofreading for English errors!
I am a (former, unfortunately!) UK teacher with LOTS of time to read, but not a lot of money, so I decided to offer free proof reading in exchange for free reading material! I have met a marvellous group of friends that way, and so now I sometimes review and promote their books here, too! email me and I will get in touch privately. Or check out my Reviews