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.