@
pming, again just for the sake of clarity, when we're talking about a wizard casting
Shield as a reaction to a surprise attack, we're only talking about that happening
after the wizard's turn has passed. I agree with you that before and during the time that the surprised wizard is unable to move or take actions, i.e. during its turn, it is also unable to take reactions, such as the
Shield spell. That's why I and certain others interpret the rules to say that this is the period of surprise. What the rules for surprise expressly allow, however, is for the surprised creature to take reactions
after its turn has passed. Because it was surprised, "full-round actions" such as Dodge/Dash/Disengage are denied to it, because you need to take those actions on your turn. An "instant" reaction like
Shield, however, is permissible as long as the triggering attack occurs after the surprised creature's turn has passed. To me, at least, the idea is that the wizard is surprised by the initial attack, and so denied the use of actions and movement, but because of a high initiative check, the wizard recovers from the shock fast enough to cast
Shield as a response.