NameGreg said: "Lin Manuel Miranda obviously comes to mind with this topic, but who are other creative forces that have acted in their plays or musicals?"
"You can't overrate Bernadette Peters. She is such a genius. There's a moment in "Too Many Mornings" and Bernadette doing 'I wore green the last time' - It's a voice that is just already given up - it is so sorrowful. Tragic. You can see from that moment the show is going to be headed into such dark territory and it hinges on this tiny throwaway moment of the voice." - Ben Brantley (2022)
"Bernadette's whole, stunning performance [as Rose in Gypsy] galvanized the actors capable of letting loose with her. Bernadette's Rose did take its rightful place, but too late, and unseen by too many who should have seen it" Arthur Laurents (2009)
"Sondheim's own favorite star performances? [Bernadette] Peters in ''Sunday in the Park,'' Lansbury in ''Sweeney Todd'' and ''obviously, Ethel was thrilling in 'Gypsy.'' Nytimes, 2000
Christopher Durang as Vanya in “Vanya and Sonia and Masha and Spike” at the Bucks County Playhouse. Apparently he couldn’t remember his lines which is ironic.
Noel Coward was nicknamed The Master, because he was not only a great writer but a great interpreter of his own work, at a time when a double threat of that sort was extremely novel (and at a time when songwriters farmed out their work to others as a rule).
It's quite common with solo shows, especially when you look Off, and Off-Off Broadway, and places like the Edinburgh fringe. So common, in fact, that's not even worth trying to name examples. Even if you exclude solo shows, it's still so common that we're unlikely to be able to name all of the examples in one thread. But for what it's worth, I keep a list of the examples I've seen personally (I'm a compulsively list-keeper by nature), excluding solo shows:
Larry David Lin-Manuel Miranda Sting Dave Malloy Harvey Fierstein Sara Bereilles Amy Staats Cesar Alvarez Ngozi Anyanwu The Bengsons Andrew Lippa Brandon Jacobs-Jenkins Nia Vardalos Shaina Taub Seth Rudetsky Tracy Letts Halley Feiffer Kate Hammill Grace McLean
John Cariani's acted in a few productions of Almost, Maine. Stew played the narrator in Passing Strange, and got quadruple nominated at the Tonys (Best Book, Score, Orchestrations, and Leading Actor - Lin and Malloy can eat their hearts out).
Sherie Renee Scott - Everyday Rapture Shoshana Bean - Dear John Mayer Anna Deveare Smith has starred in multiple plays she has written. Bowen and Bell - Title of Show Griffin Matthews - Witness Uganda Colman Domingo - A Boy and His Soul Anthony Wayne - Mighty Real Stew - Passing Strange
For all I said above, turns out this thread is actually helping me realize there are gaps in my list: several of the names mentioned above were people I had seen, but forgot to put on my list. For example, I totally forgot that Heather Raffo played the lead in Noura.