I've always gotten the signature after the doctor has explained the procedure. What if the patient claims the doctor didn't explain something, or added something in? It is a conflict of interest for the doctor doing the procedure to be the only one to witness the consent, the patient or family could even come back and say the sig was forged. The need for another witness is important.
I've been a notary public, ALL I was doing was witnessing the signature and verifying the identity of the person signing, and having them swear the contents were true. It is not the responsibility of the "professional witness", (the NP) to know what is in the document, only that it is fully filled out and true to the best of the signer's knowledge. (Unfortunately, many notaries haven't really read their handbook, and never even ask if the document is true!
As a nurse, again all I am really doing is witnessing that the patient is truly the one signing; ethically I think there is more to it, and I won't ask for consent until I'm sure the patient 1) had the procedure adequately explained by the MD or NP, 2) is able to sign for themselves (no slew of morphine or Versed on board, 3) is actually willing for it to be done (not being pressured), 4) has had time to read the consent for themselves or has me read it to them. All of that is my job as the patient advocate. If they are incapacitated, then next of kin or POA signs. If there are narcs on board, I ask family to also sign sometimes. I work in a busy ER, but there is always time for those steps unless it is an immediately life or death situation, in which case we don't do paperwork first!
(Verbal consent if possible.)