We hear so much these days about the star system and Broadway. We all know that big names from movies and TV have distorted the expectations of theatergoers. We worry that too many pay too much -- at premium mega-prices -- for the chance to see a live celebrity in an exclusive limited run, and we lament how many producers are more interested in whom they cast than what they're putting onstage.

This is not that story.

Instead, this is a chance to marvel at the number of wonderful no-star shows that have flourished on Broadway lately -- plays and musicals that managed to create their own stars by virtue of pure theatrical quality.

And yet, in unexpected ways, the no-star hits have become victims of their own success. They have made their own stars, which is terrific, but, increasingly, the new stars don't perform at all performances. With audiences expecting to see the artists about whom everyone has been raving in hot-ticket shows, theatergoing has become even more complicated.

For example, someone coming to see Helen Mirren in "The Audience" would be supremely disappointed to learn that someone -- labeled an understudy, a standby or an alternate -- was there in her place. It would be less crushing to get to the theater and find out that Robert Fairchild, the remarkable ballet dancer who is the season's breakout must-see performer in "An American in Paris," is not at your performance. Similarly, young Alex Sharp, straight out of Juilliard, who won the best actor Tony for his astonishing, extremely physical portrayal of an autistic boy in "The Curious Incident of the Dog in the Night-Time," does not work for two out of the eight performances. This is not disclosed on the play's website. And though Lin-Manuel Miranda hasn't been an unknown since he composed and starred in the 2008 Tony-winning "In the Heights," his absence from the phenomenally successful "Hamilton" feels especially significant.

And here's my point. President Barack Obama and his daughters didn't get to see Miranda onstage as Alexander Hamilton when they famously saw the show last month. The artist, who wrote the book, music and lyrics, did not perform at the July 18 matinee.

He had previously scheduled himself to fine-tune the Off-Broadway smash for the official Broadway opening and watched from the audience that day. Obama -- even Obama -- wasn't enough to get that changed. Javier Muñoz, who plays Hamilton on weekend matinees, performed the role.

Perhaps you can relate to what is, at least momentarily, a letdown for audiences who realize they are about to see an alternate or an understudy.

Careful shoppers can read about replacements on the websites for "American in Paris" and "Hamilton" before they commit. Fairchild is replaced by Garen Scribner, a former soloist with the San Francisco Ballet, on Wednesday nights, Saturday matinees and during Fairchild's vacation Aug. 11-16. Co-star Leanne Cope is off Wednesday matinees, when Sara Esty, a former Miami City Ballet soloist, is on.

Ballet dancers, of course, would never perform an emotionally and physically exhausting full-length work eight times a week. Their bodies are simply not trained for that. Nobody expects it and I was amazed that Fairchild and Cope were able to manage it so beautifully since the beginning of previews in March.

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Chris Boneau, the press representative speaking for the producers, told me in a statement that the two "have always wanted and intended to do eight performances per week, but it has become increasingly clear that the roles are just too demanding to ask them to continue."

The role of Christopher in "Curious Incident" is also exhausting. Sharp, who leaves the production Sept. 13, has been onstage for six of the eight performances since the play began previews last September. According to a press representative for the Tony-winning drama, director Marianne Elliott decided before the London premiere (with a different cast) that Christophers in all the companies would only do six. Taylor Trensch performs in the other two. Since weekly schedules change, however, the website doesn't spell out which Christopher you're buying. I find that troubling. (Most of the entire company will change Sept. 15.)

The new no-star stars are bumping into a problem that is familiar to big-brand stars -- the difficulty of performing eight times a week. In 2000, when Carol Burnett only wanted to do seven performances of the Sondheim revue, "Putting It Together," the producers hired Kathie Lee Gifford to do the eighth.

The bigger the star, usually the fewer the performances. And the bigger the star, the less chance producers can plug in satisfactory replacements to fill out the eight performances generally believed to be necessary to make a profit. The star might just be working seven, but all the union employees in the theater get paid for eight. In other words, the bigger the stars, the higher the ticket prices to pay their massive salaries and to make up for that extra day.

"The eight performances are important to the tradition," says veteran producer Emanuel Azenberg. "You have to raise the price, which makes it almost inaccessible for young people."

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Last January, Al Pacino seriously joked to The Associated Press that he "wouldn't do eight performances if you paid me. It's too much. I gave that up a long time ago."

In 1984, Dustin Hoffman made headlines for refusing to extend in "Death of a Salesman" beyond June 10 unless he could do seven instead of eight. He said the physical and emotional demands of Willy Loman were just too much to do two shows on Saturday. "The main thing I want to say, from my heart, is that I'm thinking of the audience," he said sympathetically. "They should not see a grade-B performance."

Bernard Jacobs, the late president of the Shubert Organization, responded that "the more often you let someone do less than eight performances a week, the more you set the precedent, the more you weaken the fiber of the American theater." A compromise was reached. The production closed July 1, Hoffman took a break and returned in mid-September for two more months of eight a week.

Let's talk history. Lee J. Cobb, who created the first Willy in 1949, did all eight. Hearing about Hoffman's complaint, Helen Hayes, then 84, told The New York Times, "I don't know what the problem is. Have people gotten weaker? If they're not up to it, they shouldn't do it. Of course, eight performances a week is difficult. But when we got a good position in the theater, we knew what was expected of us and we never questioned it."

Questioning is not necessarily a bad thing. Who hasn't wondered, after a daunting experience in the theater, how the actors could possibly do the whole thing again -- twice a week in the same day? As Broadway attracts actors from TV, movies, even the dance world, traditions may not hold and non-star productions have new requirements. But for theatergoers, more than ever, the tradition includes buyer beware.