CHICAGO (CBS) – It is a disturbing moment at one of Chicago’s celebrated theatres.

Stacy Keach, the star of a one-man play at the Goodman Theatre fell ill on stage and the production ended suddenly on Tuesday night.

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CBS 2’s Jim Williams was at the performance. The audience, including Williams, were shaken.

“I’ve seen a lot of theatre,” Williams said. “But nothing like what happened last night – with only about 20 minutes left in the play, an announcer stops the performance asking Stacy Keach to leave the stage.”

It was opening night for the play.

As seen in an earlier performance of Pamplona, Stacy Keach portrays Ernest Hemingway with gusto.

But Tuesday night, the play’s official opening, Tribune theatre critic and CBS 2 contributor Chris Jones could tell something was wrong.

“The clue came when you began to hear lines of text repeated — over and over — over and over again,” Jones said.

What confused some members of the audience was that Keach was playing Hemingway near the end of his life, when he was struggling to write.

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“A lot of the audience thought this was planned, an attempt, least in the first few minutes of the show, that this was Keach who’s one of the great American actors,” Jones said.

But an hour into the play an announcer on the PA system made this alarming declaration: “We are having technical difficulties. Mr. Keach, please leave the stage.”

Keach walked off and the play was over.

“Human being are mortal. Actors are mortal,” Jones said. “Things go wrong in live theatre. I’ve never seen anything quite like it.”

Robert Falls, the play’s director and the Goodman’s artistic director, came on stage to say Stacy Keach had been sick all day but insisted he go on.

Jones calls Keach courageous.

“There’s a generation of actors for whom this is like a spiritual commitment. They are determined not to stop the show,” Jones said.

The Goodman just announced Wednesday that Stacy Keach, who is 75, is undergoing medical tests and Wednesday’s performance is cancelled.

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The hope is to continue the play’s run, but it depends on Keach’s health. The play has no understudy.