With both proving to have a nice line in comic timing. Judge Reinhold & Fred Savage play father and son respectively and both do considerably well with the material to hand. The amazing thing is that this film was written and produced by Dick Clement & Ian La Frenais, the people responsible for the Ronnie Barker comedy, Porridge. From the moment the switch occurs, however, hilarity ensues, and both Fred Savage and Judge Reinhold nail their reversed roles. Before the switch, Fred Savage is every bit as awkward as you can imagine in his role as Judge Reinhold's character's son. You've got Fred Savage, a kid who - to my knowledge - never, ever came off as a kid in anything he ever acted in, and you've got Judge Reinhold, a guy who generally just always has a certain youthfulness to him, both cast in another take on the 'Freaky Friday' formula.
I don't think they could've cast this movie any better.