When someone says they like to observe people they probably do but may find you suggesting they take a test or take it to another level an imposition.. Could also be they are interested but not THAT interested. Maybe you are misinterpreting the degree of their liking. Also, a lot of people are interested in doing things but they'd rather do it on their own time, you know take their own sweet time. Also, mood is a factor.
People who are really into something are likely to be quite discerning about it. Lots of dog people don't like certain breeds, for example.
I think in interested in my interests.. But it does not mean I'm in the mood for em every second if the day..