Vivienne wrote:on a trip to Italy, when we had to be diverted, owing to lightening (not safe to land where we were supposed to)... of all the things going through my mind, painting toenails simply wasn't there. I remember quite specifically muttering over & over to myself, "why can't you go anywhere without some incident".
In my experience, people tend to worry far more about what might've happened and what might still happen than about what has happened. Now, in an extreme event such as a plane crash then you could very well be a complete wreck, but most of the worries are over; you dont' have to worry about what could've happened - you've just been in a plane crash, it couldn't get much worse - and you don't have to worry much about what will happen, because you think that the worst is over.
In the first episode of Lost we do see Shannon screaming her head off while the plane wreckage burns around her and other people are lying around trapped, bleeding or dead. It was a couple of days later that she was painting her nails (I think, I could have the time-line wrong on that, it wasn't exactly an important event...or was it?). Shannon was being particularly useless in the early days, she was sunbathing and, as you saw, painting her nails - she was expecting to be rescued soon, she was just trying to pass the time, and she didn't have many skills that were particularly useful post-plane-crash. She did have a skill that none of the other Losties possessed, but at the beginning it isn't obvious that she either possesses that skill or that it will be in anyway useful.