I ran an Acquisitions, Inc. campaign where a Beholder named Mr. Thrakozog disguised himself as a mild mannered human by wearing a trench coat, a fedora, and a fake mustache and hired the group to recover some artifacts for him. Most of the NPCs saw right through his disguise but played along because, well, when a Beholder wants to pretend to to be a human you just play along. The monk was the only PC who failed to see through the disguise and I would just describe Mr. Thrakazog as a rather rotund man who stared at others with a great amount of intensity. Granted, this was a rather silly campaign.