It is easier to get a creative person to explain their creativity in the context of an actual creative event. Describe the kind of creativity that interests you then seize from their recollections the event most likely to touch your interests.
The right person in the right bar can dig this deep without effort. We suggest a slightly more formal method and save the bar for careless conversation.
Things to Try
Imagine a specific kind of creativity. Find a way to describe it to others. Ask them who they know that does this regularly. They are your subjects.
Launch a project with a goal. Make it simple, like a talk or post. Create a lasting artifact from your notes. Be prepared to organize these in a way that they will be accessible to others without the work you've put in.
Approach creatives with respect and humility. You will need 90 minutes of their time. Make it clear that you will use it well and that lasting value will come from it. Prepare a brief orientation note. Include it in appointments.
I am collecting notes on advanced coding techniques used here at New Relic and elsewhere. I would like to interview you in this regard. I can imagine two subjects: 1. Often overlooked, but very useful techniques. Things you will try before others do. What about our environment makes them useful? 2. Influences from other disciplines that guide your work. What principles of science prove especially useful? How can others taste this without practicing science?
Feel free to deviate from the subjects you suggest in an invitation. Listen for passion as you probe for some event to be the centerpiece of your conversation. Go deep when you have one you both like.
Develop a chronology on the whiteboard. Use a few loaded words for each step. Check that these words fit. You will be showing what aspect of their work interests you. Their work need not be finished to be of use to you.
Reflect together about the map you have made. Ask probing questions about sequence, cause and effect, fear and joy, clues observed and missed, experience that proved useful. Only now admit that you have had similar experiences. This is you preparing to organize insights you gain into a coherent work.
Why It Works
When creatives are in the moment very little attention goes into how they will make decisions. Decisions just come to mind.
Recalling an event recreates enough of the moment that decisions will again come. Reflecting, the question isn't is the decision right, but is the decision consistent with ones other memories. This provides a porthole to the creative's tacit knowledge. wikipedia 
This method has roots in protocol analysis and specifically retrospective think-aloud protocols. wikipedia