I was an enumerator for the 1981 and 2001 census.
1981 recruitment was very casual.
2001 was much more 'professional' with application form on which you had to state your experience and suitability.
Passed that part and was invited to an interview with regional and local organisers. Most difficult question was 'what would you do if someone refused to fill in their census form?'
The answer is that they can chose not to fill it in however you should inform them that failing to complete a census form is a statutory requirement that may result in legal proceedings.
I shan't do it in 2011. the questions are now too numerous and too probing. There was much more hsotility in 2001 than I experience in 1981.
Be prepared to have to sit down with some people and help them fill in the form. the daftest experience I had was filling in someone's form while he was in the shower and shouting the answers through the bathroom door. He lived at home and although his parents had completed their details he wouldn't give his to them but didn't mind giving them to me.
I also had one chap running up the road to hand his to me as he had been divorced but had failed to tell his current partner this (who thought he was still married) and he didn't want her to see the form.
Also helped a very young mother who was illiterate fill hers in as her partner who normally dealt with these things was in prison.
went back 4 times to one house to collect the form having made an appointment to do so with the householder. Each time it hadn't been completed. gave her a warning that this was the last time I would try to collect and which prompted her to run after me screaming that I was being unreasonable 
Oh yes - it's an experience 