Alan Alda was an occasional panelist on the color remake of What's My Line, with Larry Blyden hosting for much of the run. I do remember seeing an episode where the two of them talked briefly about their time together in The Apple Tree. Jack Cassidy was also a panelist at times. (The two "anchors" of course were Kitty Carlisle and Soupy Sales.) Not infrequently, the mystery guest was a Broadway star - and sometimes they would wind up discussing the show they were currently in, or a past glory.