The witness is an objective and uninterested third party who is completely sound of mind and trusted by the party or parties in question. The Law Dictionary relates that the "ideal" witness has known all involved long enough to provide verification that each person is who he says he is.
A good witness also has no financial interest in the situation at hand. It is common to use a notary public, a person who is certified by the government to place his signature as an official witness on the documents with a special stamp. If a person has been a customer in good standing with a particular bank for a long time, a bank officer is also permitted to witness legal documents because the bank is potentially held liable when fraud is discovered.