Unlike a standard holographic projector, the interactive version could project holograms which responded to and interacted with the environment around them. This capability, however, made the devices significantly more expensive than a standard projector.
In 2374, Quark cited the high cost of an interactive holoprojector as the reason his Morn hologram could not speak. He also said, however, that it was good not to have to listen to the normally talkative Morn while he was away from Deep Space 9. (DS9: "Who Mourns for Morn?")