Take a look at http://www.mediacollege.com/video/camera/focus/depth-of-field.html for a quick but illuminating discussion of DOF and how to (try to) control it.
One of the easiest means of controling the appearance of DOF in an interview is to keep light off the background, thereby pushing the subject into the foreground.
An Australian DP I read about used this trick in a tight situation: he had to shoot a talking head at an outdoor stadium. The client wanted us to be aware of the stadium but didn't want it to take focus. The DP brought in a big piece of fine mesh netting, the plastic kind used for window and door screens and hung that, stretched tight, behind the talent. This effectively grayed out and softened the background, making it appear to be outside the DOF.
I'm not sure why you feel you have to "blur out" the background, however. Why not pproject some kind of soft pattern on the blank wall and shoot the talent in front of that? We often use a small Venitian blind, hung low on a "C" stand and off camera, with a light shining through it to cast soft shadows on a wall, or cut random patterns in a sheet of foam core and throw soft-focus light from a Fresnel through it to the wall. You can put color media in the holes (gel) or in front of the light to achieve lots of interesting background effects. With the intensity of the light kept low, these patterns break up the wall surface and creat the illusion of DOF.
DP Walter Graff has an excellent discussion at http://www.bluesky-web.com/backgrounds-help.htm, with photo illustrations of lighting backgrounds. His website is an really excellent source of information about the techniques and technology of our art and craft (http://www.bluesky-web.com) Be sure to look at the "Instruction" section.