If we and the Runner can both perform functions (us rezzing a card, or the Runner using a card effect, for example) at the same time, the Runner always gets the first opportunity to perform any functions he or she likes, and then we perform any functions we like. The Runner does not have an opportunity to respond after we perform our functions. However, effects whose function is to prevent other effects are used whenever appropriate, even if it is the other player's turn to perform functions. For example, at the end of a run, the Runner has a card in play that allows him or her to search his or her stack for a program and install it. The Runner decides not to use that card, and then we rez an upgrade that deals Net damage to the Runner. It is too late for the Runner to use the card to find a program to prevent the Net damage, but if the Runner already had a card installed that prevented Net damage, he or she could still use that card.