I realised years ago that getting paid per hour is anathema to my enjoyment of work. I'd much rather charge per project (even if that charge is based on an estimated number of hours) then spend whatever time it takes to complete the project than work at an hourly rate and find myself staring at the clock.
Opposite for me! In my experience, being payed a fixed sum for a project requires a very detailed spec up front and still may lead to bickering about whether any issue is a bug or a feature request. Being paid by hour makes it possible to continuously adjust the project without either party getting screwed.
Typically employed developers get paid per time (i.e. hourly or monthly salary). Contractors may be either per hour or a fixed sum to deliver to an agreed-upon specification.
With scriptwriting (and often art in general) you first develop the "product" and then try to find a buyer. This is IMHO not a feasible model for individual software developers.
