So, how does the question of whom you are interviewing with fit into the equation? First, you need to answer this question. This question will rarely be asked at the beginning of the interview, though. Before you ever answer this question, the employer you are interviewing with must—and I mean must—know that they are your first choice. If the employer thinks this, then telling them that you are interviewing everywhere can help you.
Back to the situation with the 15-person computer firm. You could still very easily get an offer from this firm if you play your cards right. First, you need to walk into this interview and convince the firm that you really want to work there.
Maybe you know someone at the firm that has said good things about it.
Maybe they do a certain type of computer work you've been interested in since high school.
Maybe their office is right across the street from your house.
Maybe you want to work in a smaller firm so you can feel a more collegial atmosphere.
You need an arsenal at your disposal to give the employer compelling reasons for hiring you. If you give the firm enough reasons why you're a good fit, they will look at the fact that Google is interviewing you as something that verifies your worth in the market. The small firm needs to think you will be their first choice over Google. You taking an offer from them over Google will be a major vote of confidence in the small company, which is something they will use to impress upon its employees that they offer a great place to work.
When I was interviewing candidates for the University, I would answer the question of whether or not the candidate was really interested in my opportunity in several ways. For example, if the student had scored a 1580 on their SATs (almost a perfect score), was captain of the football team, student counsel president, and first in their class, and my school was the only top school they were applying to, then my job was easy. The student was most likely interested in what my school offered.