It is advised that visa applicants bring along a CV to the interview.
There is an International Visitors' Office associated with the National Academy of Sciences (NAS), that is very happy to help if visitors for scientific work find themselves waiting longer than the advertised 21 days. They WILL follow up on any request even if people are NOT associated with NAS, etc., as long as it's a visit for scientific purpose.
Visitors who are waiting longer than the stated 3 week period (one month max), should go online to the NAS international visitors’ office web site http://sites.nationalacademies.org/pga/biso/visas/
and fill out the visa questionnaire. This allows the Visitors Office to contact the State Department directly via the channel they have set up. They will let the visitor know what the status and reason for the hold up is. It is best for the visitor to do this themselves rather than have it done for them by a users office or conference organizer.