What should I do if I witness sexual harassment?

This public service message of the Department of Homeland Security applies not only to unattended backpacks in a public place; it applies to unwelcome sexual advances, requests for sexual favors, and other verbal or physical conduct of a sexual nature that has the purpose or effect of unreasonably interfering with an individual's performance or creating an intimidating, hostile, or offensive environment.  In other words, sexual harassment.

Under the law, supervisors may have a special responsibility to report incidents or allegations of sexual harassment to their employer.  This responsibility should be made clear to them.  Many workplaces, and some states (e.g. California's AB 1825), require periodic training of supervisors (e.g., faculty who supervise undergraduate or graduate research) about their reporting responsibilities.  That is not what I'm writing about today.

I'm writing about Bystander Intervention.

If you saw rape occurring, you would very likely call the police.  But what if you see a couple smiling at a party, and later notice the drunken couple stumbling outside with muted protests coming from one of them?  Would it be appropriate to go up to strangers, or close acquaintances, and ask "Is everything alright?"  Or to go to the host/hostess of the event and say "I have concerns about something I've seen"? What if the party is at your boss or supervisor's house, or the situation arises in a lab at night without alcohol?  What if one of the members of the couple is your boss or supervisor?

And what if it is unsafe to say something directly to the participants?  What then?

I don't have all the answers and would welcome reader feedback as to what they would do, have done, or wish others had done on their behalf under such difficult circumstances.

For additional reading:
Northwestern University Sexual Harassment FAQ
NASA Goddard Spaceflight Center Sexual Harassment Booklet
International Labour Organization Jakarta Office