Meeting a Japanese Idol

Hi, i've recently seen one of my favorite idols (Arisa Komiya) while in Shinjuku Station. But when i saw her I was too frozen up to do anything so i kept walking, but when i told my friends about it they were saying "you should've asked for a picture!" Or "you should've ask for an autograph!"
Now thinking about it is it taboo to do that in japan? Or like will they actually talk to you. I think they'll be more likely to give an autograph,but what about a picture?
I think it may depend on their agency or whatnot?

It's not taboo. But some idols might prefer being left alone when they're not working.

You are free to go up and ask "(his/her name) desuka?" and if (s)he answers yes, you can say that you're a "fan" and ask for a "photo" or "sign (autograph)". But if the idol says (s)he cannot offer you that, you just have to smile and give up. Idols often say on TV that people kind of say things like, "I love your show" as they pass by on the street, so that's another good idea.

Meanwhile, people in the cities such as Tokyo get kind of used to seeing TV people, and they do tend to leave them alone. One of my favorite actresses once sat right next to me in a crowded train with her arm squeezed to mine. I was so nervous I couldn't say anything to her. I thought it might be creepy for her to learn that an older woman she has her arm squeezed to is her fan, but I guess I could've spoken up!

I see, thanks