Hello guys...! affidavit of support is required for married applicants and they don't usually tend to ask single applicants about it but who knows you might be one of those rare cases where they ask about it so try to secure one on ur interview date ...
P.s: Don't completely relay on my informations
Firi is right, it is not required per se, however they might ask you to provide it if you fail to persuade a consular officer that you are capable of supporting yourself, i.e. have enough money in the bank for settling and possess the skills to be able to secure a job when you arrive. For example, they most certainly will ask for affidavit a person with wife and kids, who does not speak English and does not have any savings.