They weren't asked the questions.If she was illiterate the schedule was filled in by someone else, collected by the enumerator, who then copied what was on the schedule into his enumerator's book. It is a common myth that a census enumerator knocked on doors and asked who was present, and then wrote down the details, often miss-hearing, or miss-spelling. When the enumerator collected the schedules all he had to do was to to examine the schedule in order to satisfy himself that it had been correctly and completely filled up.
Nearly always, perhaps. But, to be fair, it may not have been 100% like that - the instructions to enumerators for 1851 states
"If, on inquiry for the Schedule, it is delivered to him
not filled up, he must fill it up himself, asking all necessary questions."
It was my assumption that the enumerator went knocking on doors. But it doesn't detract from the fact that if they were illiterate, the 'someone else' had to interpret what they said and write it down.
They must have knocked at the door to ask for the schedule!
See also the fascinating article re the finding of a box of surviving Shropshire 1841 census schedules
http://www.balh.org.uk/uploads/tlh-downloads/the-local-historian-43-02-may-2013.pdf(page 96) Did householders complete their own HSs?
Interestingly, of the quarter or so of schedules that were not completed by the head of household, the author estimates that most were filled in by the enumerator.