I want to impress my interviewer that I'm interested, but I don't feel that an over-enthusiastic declaration will help my chances. How can I let them know without coming off too strong?

Ignore the fact that you're nervous and answer questions in a way that provides an answer and leads to you asking the next, logical question. If the construction interviewer asks you how you feel, answer in positive terms and ask them if the room you are in is the same room in which you'll be working; or, ask them how they feel and if they like working in the construction employer; or, ask them what, in their opinion, is the most important part of the job you're being hired for. Notice that the last one includes the assumption that you will be hired.