It had been a long day covering Mr. Trump as he squired Prime Minister Shinzo Abe of Japan around Mar-a-Lago, his Palm Beach, Fla., playground, in February. The White House had finally sent reporters in the traveling press contingent the good-news announcement that there was a “lid,” the indication that the president is done moving around for the day and no more public appearances are expected. I headed to dinner, happy to be free of cramped motorcade vans and dungeonlike filing rooms, including one that day where Mr. Trump’s staff had covered the windows with black plastic to obscure our view of him on a golf course.

No sooner had I lifted fork to mouth than an email popped up on my phone: “URGENT 9:30 P.M. CALL TIME.” It was 9:27 p.m. The lid, evidently, was off.

I rushed to rejoin Mr. Trump’s press pool and we piled into S.U.V.s that raced to Mar-a-Lago. On the way, White House sources told me that the president had abruptly decided to join Mr. Abe in a public statement about a missile launch by North Korea that day. Earlier, Mr. Trump had refused to say a word about the test, but this was a president who had already developed a reputation for sudden reversals.

The scene was unlike any other presidential news conference I have experienced. As the Secret Service put me through its security check — waving a hand-held metal detector over me and inspecting my bag — I watched as a long line of Bentleys, Rolls Royces and Maseratis snaked through the driveway, bearing Mar-a-Lago members in black-tie attire who gawked at the reporters as we gawked at them. It was Saturday night in Palm Beach, after all.

We passed a wedding huppah of orchids and roses as we were led into the ballroom where the two leaders were to speak. The loud bass of wedding dance music was audible down the hall. The president emerged, grim-faced, to express solidarity with Japan as Mr. Abe condemned the missile launch.

Photographs emerged later of the commander-in-chief making a cameo appearance at the wedding down the hall, smiling broadly with the bride and her bridesmaids, and giving a thumbs-up.