I think that FSP made an interesting point here about how much time scientific types spend "on the job" when part of that job involves thinking and is not necessarily reflected in face time at an office or lab.
I tend to fret a lot before a big job. This may look like a whole lot of needless energy, and maybe it is, a little. But what I'm doing when I'm worrying is going over all the contingencies and trying to make sure I have everything covered. I'm naturally absent-minded, so what I try to do is always write things down. I have a tendency of think of all the things I need to do just as I'm starting to drift off to sleep at night, so I pull out the old pen and paper and write it down, and then I know I've got it covered and I don't have to worry about that particular item. Of course, in the morning I end up with this weird unintelligible chicken scratch because I was writing in the dark and I was half asleep.
When I was in consulting, billability was paramount. I was paid overtime (which, as I mentioned previously, is not necessarily true for all consultants) and I rarely had a problem being billable. But if I spent a significant amount of time out of work planning stuff (not just worrying over a few details, but actually figuring things out), I did bill that time. I'm not talking about a lot of time; just a half hour or so the weekend or the night before the big field project. Same thing with dinner in the field if we actually had a productive pre-food arrival conversation and weren't just bitching about whatever.
What different people consider to be billable varies. It may sound as if I bill the client for every stray thought, but my productivity was generally a lot higher than certain folks who put in more "face time", even though they were standing around the proverbial water cooler for a big chunk of the day. Everybody works differently, so I think a fair amount of lattitude is required as long as the work gets done within reasonable time and financial constraints. But maybe where I worked was an exception...