I used to use this technique in my employment, and with my own manager. Every now and then he'd float by and tell me to do something I didn't usually do - I used to say with a smile 'Sure, no problem - what would you like me to NOT do instead?'
He used to look flummoxed, and I explained that of course I couldn't fit in MORE work....unless he hadn't been giving me enough work to fill my working day in the first place (which of course he had!).
I can remember once in my youth doing secretarial temping, and doing all my work and then filing my nails - they didn't like it, but I simply said 'I've done what you gave me, and if you don't give me more, then I might as well file my nails'. The other secretaries used to tell me to 'look busy' even if I wasn't. I changed to writing letters to all my chums instead, so I 'looked busy'.
(Another place I worked told me to always make sure I claimed a certain level of expenses every week, as they did, otherwise if I didn't, it showed those expenses weren't really necessary in the first place'.....)