Many stories involving Nurikabe exist from the past, wherein much foot traffic through forests and other confusing areas made it difficult to navigate.
Many living things, humans included, usually sense walls and barriers and navigate around them, even invisible ones such as Nurikabe. Due to this, many who encountered Nurikabe would instinctively navigate around him and find themselves even more lost than before, or they might be forced to take a much longer route than they initially would have chosen.
Nurikabe is rarely seen, and it is said that he usually doesn't make a habit of appearing to anyone. Whether this is bashfulness or simply reluctance to been seen, or perhaps that most humans could not perceive him properly, is unknown. His appearance is said to be similar to any stone wall, although tales vary as to whether or not he has eyes or limbs; some say yes, many, to both, and others say that he is essentially a wall that somehow moves from one point to another and has a consciousness about him.
Although some tales insist that Nurikabe is sticky, and if contacted the victim will be unable to extricate himself and become a part of the wall, this is not consistent with other tales that say Nurikabe can simply be navigated around or through by a sufficiently resolved person. The 'sticky' story likely derives from a feeling of lack of control over one's own path, being lost, but invariably the wanderer finds himself free from the influence. The assimilation factor is likely a horrifying exaggeration for storytelling purposes. It is more likely that Nurikabe, like many youkai, is rendered harmless and mischievous simply by either a respectful and well-intentioned approach, or by a resolve that ignores the nagging urge to try and circumnavigate the wall that doesn't seem to be right in front of your path.