Many autistics have a special interest or obsession that revolves around objects or factual topics. Some may be surprised to learn that a subgroup of autistics become fixated on people or a particular person. People-watching then becomes their "obsession" or "special interest."
A female client once expressed that she loves researching facts. This will often start with researching an enjoyable movie or book. However, the rabbit trail of research quickly focuses on characters (fictional) in the story, their creators, and details of the lives of the authors and writers who produce the story. Alternatively, the rabbit trail may become focused on real people associated with the movie, such as the actors and details of their lives.
Another family shared that their female adolescent becomes obsessed with certain people she knows (generally other girls or women). The teen wants to check on where the person is at all times. She mimics and attempts to emulate her. She also adopts the interests and opinions of that individual as her own.
As described by Judy Eaton in A Guide to Mental Health Issues in Girls and Young Women on the Autism Spectrum (CHP 2, 2017) "...girls tend to display less of the more obvious obsessions and restricted interests seen in boys. Girls on the spectrum can often become fixated upon certain people or celebrities... It may result in the girl with autism wanting to collect memorabilia or facts about someone, or wanting to copy their look or mannerisms."
The pattern of fixation "on people" may be confusing for clinicians and families, with autism being ruled out as a possible diagnosis because of the strong drive toward "social" connections. However, the social approach behaviors of the individual are obsessive and inappropriate. This autistic individual fails to read cues from the other and to modify her behaviors based on the needs of others plus the social context. Additionally, there is often a lack of mutuality in these connections (when they do exist in person). It makes most sense to look at the big picture of neurologic characteristics and to view the focus on people as falling into the "intense interests" category in the diagnostic criteria.