There's a good reason Emma Watson won't take a selfie with you.

Don’t take it personally, but should you run into her on the street any time soon, Emma Watson will not be posing for a selfie with you. Not if you ask nicely, and not just this once. It’s a blanket rule, but with good reason.

“For me, it’s the difference between being able to have a life or not,”Watson told Derek Blasberg in a recent Vanity Fair interview.

“If someone takes a photograph of me and posts it, within two seconds they’ve created a marker of exactly where I am within 10 meters. They can see what I’m wearing and who I’m with. I just can’t give that tracking data.”

Having fought hard for her right to privacy over the years, the Harry Potter star says she's happy to offer an autograph, or talk about the famous wizarding world in which she starred, knowing how much the franchise means to so many people around the world.

“I have to carefully pick and choose my moment to interact,” the 26-year-old says during the cover story interview.

“When am I a celebrity sighting versus when am I going to make someone’s freakin’ week? Children I don’t say no to, for example.”

And while Watson is not naive to the reality of stardom, she argues social media has ultimately been a game changer no one in the industry could ever have predicted.

Listen: Zoe Foster Blake talks about strangers asking for selfies with her son, Sonny. (Post continues after audio.)

“People will say to me, ‘Have you spoken to Jodie Foster or Natalie Portman? They would have great advice for you on how to grow up in the limelight,’" she explains.

"I’m not saying it was in any way easy on them, but with social media it’s a whole new world. They’ve both said technology has changed the game… Privacy for me is not an abstract idea.”