I only make custom lures for friends, but I design additions to existing homes which in a sense is the same. JF is spot on about digging deep. I try to at least set up 2 initial meetings. The first is a meet and greet, get to know the person and then slide into what the customer is looking for. This initial meeting will only be about a 10-20 minute meeting.
I'll then go back and sketch something up and will write down a series of questions or thoughts I have while doing the sketch. This will become the document I use for our next meeting agenda where I'll be able to at least extract 75% of the needs and wants from that customer.
The 2 meetings may sound like a prolonged process, but at least you can get a foundation before you start putting in the bells and whistles which will take their ideas and expectations to the next level.
Unfortunately, custom doesn't mean generic.