In Shawshank Redemption, how did Andy Dufresne dig a tunnel if there was a cell next to him. He had to go through it, yet he obviously didn't.
Andy, as both a model prisoner and a cog in the warden's money laundering scheme, was given 2 priveleges; the last cell on the end of the row (a coveted cell for Andy because of its distance from most activity, which made it quieter), and a cell of his own. You may recall that he didn't set out to dig a tunnel, it just started when a chunk of the wall fell out while he was etching on it.
As a result of his 2 privileges, he was able to dig a hole in the wall that led to the sewer pipe, and he was able to do it undetected since he didn't have a cellmate.