A standard example in the literature of Frankfurt cases is where a guy is deciding whom to vote for. If he isn't going to freely vote for the candidate Dr. Black wants him to vote for, then Dr. Black will force him to vote for that candidate. But he is going to freely vote for the candidate, so Dr. Black doesn't intervene.
Here's a funny thing about this case. Dr. Black can't force his victim to vote for a particular candidate. At most Dr. Black can force his victim to check a box, press a lever, or the like. But checking a box or pressing a lever is not voting, because the validity of a vote requires that one not have been compelled. For the same reason, you can't run a Frankfurt case where the action is a making of a promise, an entry into a contract, a marriage, etc. (I don't know if you can force someone to make an assertion.) Many of our actions are of a sort that logically cannot be compelled.