Much like military members, law enforcement officers are never really off duty as your question suggests, they are more accurately off their shift.
Police powers do not start at eight AM and terminate at 5 PM--its 24/7/365. Their oaths to protect and serve remain active—so to the responsibility to comport themselves to appropriate behaviors when off shift—maybe to a degree more than expected of the average citizen—one reason a Federal law give Law Enforcement Officers (active and retired) the privilege to carry concealed weapons in all 50 states when off shift.
NOT LEGAL ADVICE. FOR EDUCATION AND INFORMATION ONLY. Mr. Rafter is licensed to practice in the Commonwealth of Virginia and the US Federal Courts in Virginia. There is no implied or actual attorney-client relationship arising from this education exchange. You should speak with an attorney licensed in your state, to whom you have provided all the facts before you take steps that may impact your legal rights. Mr. Rafter is under no obligation to answer subsequent emails or phone calls related to this matter.
Yes, an off-duty law enforcement officer can "spring into action" if the situation arises and deserves it. Most officers always wear their badge, even when out of uniform, sometimes on their belt. If a crime is committed in their presence, the officer can and usually will act as a law enforcement officer and effectuate an arrest. If he is acting in the scope of a police officer, even when off duty, then he likely will not be personally liable. He is required to act as a normal citizen in the sense that he must follow the law, but unlike a normal citizen, he is a sworn law enforcement and often required to enforce the law if a crime occurs.
This is not to be considered legal advice nor does an attorney-client relationship exist.