Update: On March 27, Prime Minister Boris Johnson announced in a video posted on Twitter that he had tested positive for coronavirus. He will continue to lead the government while self-isolating.

Boris Johnson has spent decades preparing for his lead role, honing his adopted character, perfecting his mannerisms, gauging the reactions to his performance and adjusting it for maximum effect. Now he has the national stage and the rapt audience he always craved. His speech this week announcing a lockdown drew the biggest television audience in Britain in this century.

The problem is that he has been preparing for the wrong part. The man came to power playing Falstaff, a double-dealing, comically entertaining, shameless rogue; now he is suddenly onstage as Henry V, the wartime king whose solemn judgment, intense focus, charisma and conviction must lead his nation in a time of crisis. Mr. Johnson does not know how to play that part, and it shows. This is not a rehearsal. His careless, inexcusable reluctance to track and halt the virus earlier will have cost lives.

Throughout these last weeks as the coronavirus crisis became apparent to everyone in Britain, Mr. Johnson has been indecisive, contradictory, confused and confusing, jovial when he should be grave, muddled when a frightened nation desperately needs him to be clear. The man picked for his supposed talents as a great communicator has stumbled his way through news conferences, occasionally hitting with evident relief upon a jolly riff he finds familiar. In the rare moments when he has struck the right note, he unerringly hits a jarring one minutes, hours or days later. His switches of strategy and his lack of clarity left far too many Britons oblivious to the importance of social distancing until far too late.