It was looking awfully bleak for the Saturday evening performance of “My Fair Lady” when Artistic Director Molly Smith walked onstage surrounded by her cast to deliver some grim news.

She informed the nearly full house that flu had laid low several cast members, including Manna Nichols, who played the lead role of Eliza Doolittle.

To have an understudy step in is usually a disappointment for an audience that arrives expecting to see the stars. In this case, even Nichols’s replacement, Erin Driscoll, was absent: She had lost her voice.

Now things got interesting. Smith promised the crowd it was in for something rare, something it would heartily embrace. Although the role of Eliza Doolittle is a demanding one, a member of the ensemble, Hannah Willman, was prepared to take it on with script in hand — after just a couple of hours of rehearsal. Suddenly the audience felt treated to a giant gulp of what it meant to witness live theater.

It was the final weekend for the Arena Stage show and, with flu whipping through the cast, two performances already had been canceled. Other regulars missing from Saturday night’s show were James Saito, playing Alfred Doolittle, replaced by Bev Appleton; Kurt Boehm in the role of Jamie, replaced by Victor J. Wisehart; and Sherri L. Edelen in the roles of Mrs. Pearce and the Queen of Transylvania, understudied by Jennifer Irons.

“It is the beauty of the theater,” Smith wrote in a later e-mail. “We all pull together as a company and with the audience to make theater between us.”

As the show began, the cast seemed to stamp a little harder than usual and twirl a little faster, determined to carry the brave new lead on their shoulders. But Willman needed no hoisting — she leapt into the role with confidence and stunning vocal precision. After she led the ensemble in “Wouldn’t It Be Loverly” the house burst into rousing applause, and again the response was resounding when she nailed “I Could Have Danced All Night.”

“She was navigating the role with a great deal of ease and played it as if she’d rehearsed it — this is major,” Smith wrote. “She is skilled, talented and intelligent — all those qualities shone through.”

On this night, a new depth emerged in the tale of Eliza Doolittle. In a moment of life imitating art, actress Willman blossomed not unlike Eliza herself, and the audience watched like surprised Henry Higginses as the young woman with the big heart and the lovely voice showed that she is star material.

With Willman dancing and singing, script clenched between thumb and forefinger, the evening became an unforgettable moment of theater — all the more so when, during the standing ovation for her at play’s end, the young stand-in playing a flower girl was handed her own bouquet for a job masterfully done.

Smith had to miss the grand ovation. She slipped out before the final curtain to go home and nurse her own sore throat. “What a wild way to end,” she wrote in the e-mail, praising the company for its brave performance. “I’ve never had this experience with any production. Ah, the theater, it’s a wonderful tribe we have that can pull together and make magic even in the middle of a difficult situation.”