In my experience most people who follow the big fad of the day (royal wedding, big brother, american idol, etc) are never really fascinated by it. If you ask them "what fascinates you about american idol?", the answer you're most likely to hear is "wtf, what do you mean?". Many of these things are designed to be attention-grabbing and to encourage chattering about their consequences, even if there is absolutely nothing interesting or actually at stake in that.
In this sort of situation I either (a) go talk to actually interesting people whenever someone starts blabbing about fad-of-the-day or (b) pretend to find it interesting and listen to what the person is saying only to be made bored very soon.
Hmm, my experience is quite different here. Generally I find that people may start off with some big fad of the day because it's nominally a 'safe' topic and something which can start a conversation without offending or knowing another persons interests apriori.
Almost any 'fad of the day' topic has a route into a more interesting topic.
American idol - influence of music on society, what defines popular, changes in the music industry that make idol possible, etc.
Royal wedding - the role of the monarchy in modern nation states, the role of weddings in changing fashion, is marriage as a concept out of date, etc.
Big brother - voluntary surveillance vs involuntary, peoples behavior when they know they are being 'watched' vs not being watched, the role of 'reality' tv in the definition of morals in 21st century society.
One of the things that can trip up folks is concluding that because someone just asked you your opinion on last night's American Idol decision they believe that the best use of your time would be watching American Idol. Take it as the equivalent of 'nice weather we're having today isn't it?' and you can often find a common discussion ground at the appropriate level.
In my experience, if you bring up american idol most people will more likely say something like "god, did you look at her hair??" than something like "I wonder how the perception of fame and importance will change in 21st century society after the music industry realizes the artist brand is meaningless". You must certainly have different social circles from mine, as while it is possible to talk highly about trash pop culture, most people are completely uninterested in doing so (i.e., they turn to trash to turn their minds off, not to reflect about society, life, the universe, and everything).
How do you find out if people are "actually interesting", if you never start talking to them?
You have to engage people on a personal level. Maybe like "Oh, did they vote off the wrong guy?" Something more casual, at least.
One way in which you might participate is to explore what the other person finds so fascinating about the topic. You may think its idiotic but they clearly don't, so rather than try to convince them of your point of view, find out theirs.
Typical start "Oh did you see who got voted off idol last night!?"
Response, "I haven't been keeping up, but it sounds like you were surprised by that, did you think that was the wrong choice?"
Now you have done two things, one pointed out you have nothing to add in the discussion and two not made it seem like they are an idiot for watching it.
They may have a reason behind their surprise, find out what it is, find out what kinds of music they like and why, did this person sing songs from that genre? Did they do poorly?
As long as you realize it isn't a reflection on you that someone wants to talk about something you don't seek out, you can probably find some aspects of it that are at least entertaining.