<p>"I think USC allows the different departments to choose their interviewees based on their own criteria. "
This is true. The Full Tuition scholarships are allocated to the various colleges and a students real interest in attending USC [ which requires more proof than just applying early and writing an essay saying they want to attend] is a factor in the selection process.
When DS applied to USC, he visited twice, sought out the head of the dept he was intending to major in, sent in a CD of his musical abilities[ though music was only an interest, not his intended major] and had a strong letter of recommendation sent to both the head of the dept as well as the admissions office by the scientist he was doing outside research with. That dept at USC ended up really wanting him, [his choice of major is not a popular one] and he was selected to receive a full tuition scholarship.
Showing sincere interest can never hurt , since USC these days receives thousands of early applications from many exceptional students.</p>