Shifting prominence to mark focus requires a linguistic antecedent, so if you say ‘A red apple’, chances are you or someone either said something like ‘green apple’, or there is some other reason why antecedents of this sort are salient, for example, someone might have asked: What kind of apple do you want?
Marking focus is a bit like using pronouns and other anaphoras: While there must be a proper antecedent to resolve the reference to interpret anaphoras, for focus there must be a proper antecedent to justify the contrast. Pronouns can also be used cataphorically, where the antecedent for the reference comes after the pronoun–and the same is true for focus marking.
Here’s a nice exploitation of our grammatical knowledge that focus marking requires a linguistic antecedent from September 15’s Colbert report:
Colbert is using focus cataphorically to set up an expectation for an antecedent, and then leaves us hanging in mid air. There are a number of papers at the upcoming etap conference that address focus marking and its use in context.