Other than the Candidate, there are two distinct roles when it comes to DSC2 - Approved Witnesses and Assessors. Formal assessment of the Candidate is only done by Assessor.
Approved Witness:
"The role of the Approved Witness (AW) is to give candidates opportunities to demonstrate their competence and to confirm their knowledge, and record that they have done so in accordance with DMQ set standards. "
So the AW's, who are volunteers, take the Candidate out - ideally on a single stalk - and witness the Candidate's ability to meet the Performance Criteria in the Independent Cull Report (ICR). The AW asks minimal questions of the Candidate, and only where directed to do so on the ICR. Both the Candidate and the AW then submit their respective copies of the ICR to the Assessor.
Assessor:
"The Assessor will check and evaluate the evidence against the DMQ standard and will offer advice to candidates. Assessors carry a professional standing in the area of deer management and also hold a National Assessor Qualification."
So the Assessor reviews both ICR's that have been submitted and then, through questioning of the Candidate and AW, assesses whether the evidence documented in the ICR meets the standard required. The Assessor can question the Candidate about their capabilities and knowledge, and ask for additional evidence as necessary (e.g. to complete another stalk).
Also note that, when it comes to the Assessors, "Internal Quality Assurers are appointed by Assessment Centres. They ensure that assessments are carried out fairly and thoroughly and that all Assessors are working to the same standard. All evidence is reviewed by an Internal Verifier."
You can find more information
here.