Operators do everything! :) Measure heights at entrance, push the buttons to make the coaster go (most of it is computer controlled, so it isn't as difficult as it might look), do the safety inspections, etc. If you want a lot of details on working coasters or being an employee, I have all that sort of thing on my website (below) - including the different positions for working coasters and such. Each ride is different, but they're all great fun to work!
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/