Caveat: I haven't seen the production, either with Broderick or McGrath.
However, as a standby, playing opposite the headlining female lead, he would traditionally be bound to recreate the timing and general drift that she's used to and is normally performed by the absent leading man.
Michael McGrath has absolutely knocked me out every time I've seen him over the many years, both regionally and on Broadway. He absolute pulverized me in SPAMALOT (for which I thought he easily deserved that Tony that went elsewhere)... and was the single force of nature holding NICE WORK IF YOU CAN GET IT together (for which he finally won that Tony.) He was a killer in IS HE DEAD?, and by far the best thing in TOOTSIE, with perhaps the funniest comic timing I've ever seen. Deft and dynamite in LITTLE ME (with Faith Prince), WONDERFUL TOWN, MEMPHIS, BORN YESTERDAY, ON THE TWENTIETH CENTURY.
He was originally supposed to be the lead narrator/performer in JEROME ROBBINS' BROADWAY – but something-or-other happened, and it went to another FORBIDDEN BROADWAY alumni, Jason Alexander. (I'd much rather have seen McGrath.)
The last few times I saw Broderick, he was notably... sedate, seemingly older than his 50-60 years (NICE WORK IF YOU CAN GET IT, IT'S ONLY A PLAY.)
I'm looking forward to seeing PLAZA SUITE, hopefully if I can catch it with McGrath.