Yep. I would chase them down, then beat their ass. I hate people who steal.

Okay. Yeah. Sure.

Of course!

Well , to tell you the truth , im quite timid and always think of what would happened if i chased after the person . i mean if it was a little kid , yeah than i would stop him or her . but if it's some huge guy , sorry , don't even think about it ..

Yes I would and I have.

If I'm just a random passer-by, I probably wouldn't do anything that is specifically proactive (ie chase after the person), but I would allow the "victim" to use my phone if necessary, and give a statement that I did see it happen, with any description that I have.

See #2 in my answer at http://www.answerbag.com/a_view/2459749 .

Late one night, when I was an intern in New York City I was planning on taking the subway back to my place. It was a Friday and the subways came sparingly. While I was sitting on a bench waiting for a train a drifter wandered over to the pay phone, took out a wire, and promptly inserted it in the change chute, trying to knock free coins from the mechanism. Now, you don't know me in person, but I share a lot of physical features with my namesake Alex P. Keaton (though I am much taller standing at 6 feet). This guy was as tall as me and about twice as muscular. I watched for a moment, worked up the courage, and finally commanded him to "haaaaaaa." That's the sound of the word "STOP" being caught in my throat. Apparently my exhale didn't intimidate him, he continued at his task. This guy could have gutted me right there. Finally coherent language came from my lips at a sound no louder than a whisper. "stop." The drifter continued his jolly jimmying. I was now confused as well as scared. Even if he didn't hear me he should have at least acknowledged my confrontational presence 10 feet from him. I scared myself when I finally said "STOP!" And to my utter amazement, he continued his work unbroken. I walked up the escalator to the concourse and found a police booth. I told the officer inside what was going on. And he didn't care. Of course, in hindsight, I neglected several things, including my safety. First was that this drifter was likely on a cocktail of drugs that made him immune to logic and reasoning, and very likely more willing to simply push me out on the tracks to go after some Washingtons (not the $1 variety). I also neglected to think that my wallet was just as easy of a grab and one much more profitable than his change scam. Finally, when I did decide to confront the guy, I did so putting him in between me and my only exit from the train platform. So your answer is yes. I would do something about it. Chances are I would be an idiot in my superhero motives, but yes, I would try.