Based on my experience, I'm guessing your client would rather not pay your hourly fees to select paint or carpet for their space. Since my rate typically runs less than most of the architects in town, I can assist you in finish/furnishings selection and follow through with the specifications.
I've worked both as a direct representative of the architect with the client and as an independent interior design consultant. However you prefer to work is acceptable to me. The level of attention I give to a project along with the ability to work within tight time frames are skills I bring to your project team.