Include specifics in your answer so the interviewer better understands the context for your actions. You can also talk if being firm with people is something that comes naturally to you, or if you have to consciously make an effort to do so at work.

"Yes, I am able to be firm with people. In a previous position, we had a client who was repeatedly calling for an update on their case. I was able to do a lot of the initial status reports for them, but the attorney needed to provide details. For whatever reason, the attorney was not getting back to the client. It was straining my communication with the client because he was losing trust that I was passing messages along. I had to have a direct conversation with the attorney and explain why she needed to call the client back. It was eventually resolved but it reminded me of the importance of clear communication on all ends while working a case."