One aspect of Nathan Fielder’s comedic mission — inspired partly by reality TV — has always been to see how just how far people are willing to go, particularly when they’re being watched. Could he persuade a small business to sell poo-flavored frozen yogurt, for example, and persuade customers to try it? The answers, as seen in “Nathan for You,” were yes, and yes. “The Rehearsal,” his nearly indescribable first project on HBO, at first feels like an extension of that show. Can he persuade people to rehearse real-life situations, from offering an apology to raising a child, until they’ve exhausted every potential outcome? As is always the case with Fielder, though, the insane premise is merely a launchpad for something far more peculiar, as he begins to grapple with the parts of life he’s missed out on in pursuit of his career, and the potential damage he’s caused with his docu-comedy pranks. It’s difficult to recall a more polarizing piece of television. By its final, truly difficult-to-watch episode, some will adore it, many will despise it, but everyone can agree that nothing like this has ever been on TV. — T.M.A.