Scoring candidates
We recommend you create a scoring criteria for each question beforehand and ask each candidate the same questions to compare like-for-like.
However when we get to deciding on the best candidate, we don't recommend that you simply base this decision off of the highest scores as there will be all sorts of bias that will make one person score in a different way to another. Plus your mood or context may also affect how you score from one day to the next. Instead you should view your scores as a quick way of remembering how you felt about one answer in comparison to the other, and of spotting the patterns in how each hiring panel member judged the candidates attributes and experiences.
When creating the scoring criteria you should work with the hiring panel to decide:
Diverging from the script
It's absolutely okay to ask follow up questions to your original question, or to explore something in more detail with one candidate that might be relevant to them and their skills and not others. The interview script and scoring is there to ensure there is a fair baseline and that you as a hiring panel are being conscious about why you think one person is a better fit than the other.
However when interviewers first start using scripts and scoring they can find themselves behaving rigidly and missing out on important opportunities to learn about the candidate and build a relationship.
If you diverge from the plan simply ensure you do three things: