Katie Holmes Delivers Triumphant Secret Stage Performance

Despite her casual attire, Katie Holmes' performance during a reading of the play "One Day When We Were Young" was anything but nonchalant.

According to theatergoers present at the surprise, top-secret showing in Williamstown, Mass., on Aug. 17, Holmes -- clad in jeans, a basic white top and a ponytail -- brought her A-game opposite British actor Jeremy Strong. "She capably delivered," one eyewitness tells Us Weekly. "She was very focused on the piece."

PHOTOS: How Katie has transformed

In fact, "If her part in the reading is an indication, I would say she's eager to get back to acting," the witness adds of Tom Cruise's ex.

Since filing for divorce from her husband of five years on June 28, Holmes, 33, is dead set on moving forward in both her acting and design careers. This fall, the actress is set to head back to Broadway for the first time since 2008, when she performed in "All My Sons." She will also show her first collection at New York Fashion Week with design partner Jeanne Yang. (The pair launched their label, Holmes & Yang, in 2008).

PHOTOS: Katie and Tom, the way they were

"This is a big deal for Katie to show in the tents, under the full attention of the fashion editors and the glare of the media scrutiny," a source told The New York Post Thursday. "They could have done a small show elsewhere, so this proves she and Jeanne are very serious about their business."

MORE FROM WONDERWALL:

Court Documents Show Cruise-Holmes Divorce Case Closed

Katie Holmes Teaches Suri Cruise How to Ride a Bike!

Daily Beast: Suri Cruise's Guide to New York City: Bronx Zoo, Pastis, More