The idea of type stereotypes. Be wary of them and how they may influence your typing of people.
Some basic ones:
Not all cheerleaders are ESFj or ESFp
Not all jocks are ESTp
Not all physics majors are TiNe
Not all entrepreneurs are ENTj
Not all accountants are ISTj
Not all middle managers are ESTj
Not all poets are INFp
Not all flautists are ISFp
Feel free to add to this list.
Now this doesn't mean to discredit general type tendencies which I believe are true. Because I do believe that certain areas would allow certain functions to prosper more than others, however when determining a type I think it's important to look for the patterns you see in the areas a person chooses to involve themselves, and not just use one area to say "ok he fits the mold of a XXXX". You also have to look at what aspects of their chosen areas they focus on, and the way they go about it, which ultimately indicates their functions.
For example if someone is both into theoretical physics but likes watching sports because he likes to watch and follow what strategic moves the coach/manager makes and their impact on the game, and they like physics because they like examining the different possibilites of what exists and analyzing it, you can assume the person has Ti, since relational logic is being used in both instances. However from his interest in the physical and realistic aspects of sports, you detect Se, but then from his interest in exploring the possibilities of the unexplained in physics, you detect Ne. (If in physics he chooses to focus on the applied aspects of physics, it may yield a different functional interest) So you can't quite determine from this which percieving function they use more, even though you have the judging function.
Therefore regardless of what percieving function this person has, he has a common interest with both sports fans and physicists, who fall into different socionics "sterotypes". So a person shouldn't be suprised when he can relate to or is friends with people from both ends of his functional interest range.
He with his broad range of interests should have both SeTi friends and NeTi friends (and their mirrors), because notice the common link is Ti. Not everyone's friends have to match on every function, or on any function for that matter. Relationships depend a lot on context and interest as well.
To determine which percieving function is dominant, you would have to look again at what he gets out of his two different interests. When he watches sports, is he focused more on the action itself and the play at hand (Se), or is he focused more on the overall flow of the game and the possibilities at hand at different points in the game (Ne)?. With physics is he more interested in figuring out how to use his knowledge to be practical and possibly profitable (Se), or to be focused on following the deep rabbithole (Ne)?
Introversion/Extraversion you could also see with how they go about their interests.
Just an awareness thread. Feel free to add