There is no question that it is important for a job applicant to learn about a company prior to a job interview. After all, a candidate can easily turn a job interview into an exit interview by demonstrating little or no knowledge about a potential employer.
At the same time, the personal information that you accumulate about the individual interviewer prior to meeting with him or her is a little different, but this depends in part on the position you are seeking. For example, if you are seeking a job for a tabloid magazine, your data on the interviewer may help demonstrate your interest and ability in digging up such information. And for most other jobs, some basic information about the interviewer may help you shape your comments and overall presentation.
Nonetheless, although the personal information that you gather is presumably public, it is possible to go too far. Your knowledge about a company or interviewer should indicate that you are a highly interested and motivated job seeker, and not a stalker. Knowledgeable employers keep their pre-employment questions focused on job-related topics, and for the most part, you would be well advised to do likewise. | 
