Both are married, though a source close to 29-year-old Katharine told People that she and her husband have been "separated for over six months, since early this year".
CLICK ON PHOTO TO ENLARGE
The former American Idol finalist has been married to 47-year-old Nick Cokas, also her manager, since February 2008.
Michael, 47, is married to The West Wing actress Mary McCormack, and they have three young daughters together.
So far, representatives for both Katharine and Michael have refused to comment.
According to TMZ, who first posted the pictures, the couple were holding hands and cuddling each other before the kiss was photographed.
Smash was cancelled earlier this year after two seasons.
In January, Katharine explained that her love of work meant that starting a family had been put on hold.
"I once thought that by now I'd have lots of children, but actually I'm really enjoying being able to go from point A to point B and the only worry I have is my dogs being walked," she said.
"I'm selfish right now, but that's how and where I should be. The thing is, work is the thing I love the most."
Katharine and Nick first met in 2005 when they performed together in a stage production of The Ghost and Mrs Muir. They got engaged in 2007 and were married in Beverly Hills the following February.
Michael and Mary have been married since July 2003, and are parents to Margaret, born 2004, Rose, born 2007 and Lillian, born in 2011.