A few weeks ago a friend of mine was showing me her recent research. She had a breakthrough on a difficult line by researching a witness to a marriage who turned out to be important to her family line. She had the document for some time, but hadn't thought to research that person before. This got me thinking. How many times do we overlook those people and miss critical information?
I have heard many genealogical breakthrough stories that involve looking at the next census sheet, researching a person on a document that didn't seem to be directly related, or looking in a county that the family didn't live in. These bits of information are found by doing searches that are seemingly outside the focus of the research. I like to call this "genealogical peripheral vision." Hopefully by using your genealogical peripheral vision, breakthroughs can be more deliberate.
In many of the documents a genealogist uses, there are periphery people. If you find a name on a record related to your family, and you aren't sure who that person is, it is a good idea to find out how they relate to the document and your family. Witnesses to marriages and wills are good places to start, but be creative. Periphery people can be found in other places as well. It may sound like extra work, but researching these people may lead you to the answers you have been looking for.
Because many families lived near each other, look at a few pages before and after the page where your family is found on the census. I have seen many people find a family in the census they had been unable to find before because of a horribly misspelled last name.
While researching, keep in mind families that you aren't focused on, but who lived in the same area. Keeping these other families in mind can help you spot them as you sift through information and may indirectly benefit you.
Another place to use your genealogical peripheral vision is in neighboring counties and states, especially when your family lived near a border. With the constantly fluctuating counties in the United States, this can solve many problems. For example, I found birth and death dates for an entire family when I looked in a cemetery book of the neighboring county.
Honing your genealogical peripheral vision can help you find information you weren't necessarily looking for but that is pertinent to your family line.