Fair question. Still not voting so as not to skew results, but to answer the question...
It all depends. If I am off duty but driving my work car (I have the thing 24/7, am on call, etc.) then I am not really off duty and will act accordingly.
If I am in my jurisdiction, and it is a situation that warrants intervention, and I am in a position to do something appropriate then yes. (and as noted above, that might be just calling for help.)
If I have been drinking then no, not unless someone is at risk for really serious harm. The liability to me has gone up too high if I act.
If I am out of my jurisdiction....depends on where I am and what the circumstances are.
To give some examples; bar fight...not getting involved. I will be a good witness when/if I am needed. Traffic crash; If I can get to it I will help out. Convenience store robbery; depends on what I can do safely.