The actors sang live on set.
Instead of using Hollywood actors for the main cast, producer and director Clint Eastwood cast mainly stage actors who had played the role every night on-stage.
John Lloyd Young reprised his Tony-winning role as Frankie Valli from Broadway. Donnie Kehr and Erica Piccininni also reprised their roles from Broadway. Renée Marino also reprised her role from Broadway. That's how she was discovered for this role, when Clint Eastwood saw her in the show.
Frankie's parents have a picture of Frank Sinatra in their home. Frankie often says he wants to be bigger than Sinatra. Like Frankie Valli, Sinatra began his career singing in night clubs owned by Angelo "Gyp" DeCarlo.
Clint Eastwood: Stock footage of a young Eastwood as Rowdy Yates in Rawhide (1959) is seen on a black-and-white television.