I've been in that "Do I get involved?" situation only three time somehow.
First time: It was middle school, I saw someone get bullied. I was in 6th grade, and I was confused, scared, etc. I didn't interfere and it's haunted me for while.
Second time: 10th grade, last year, I was walking down the hall with a friend and I saw the same situation and interfered. the guy was a freshman was scared shitless by me (I'm, almost 6 foot) He ran just from me approaching.
Third time: A few weeks ago, I'm with friends and I see the same situation, this time it is someone we know and chase the guy down. I had never seen someone run so fast before.
So my answer: It depends, but most likely.