With all the talented people involved in this production, I don't know how it could've gone so terribly wrong. But it did.
There are many ludicrous aspects to the plot, and other reviewers were right in saying that the pacing is very choppy, with no clear indication of how much time has elapsed between events. I was irritated by the story, which is incredibly contrived and melodramatic. First, there's no real reason for Sylvia to start dressing like a boy - oh sure, the authorities will be less likely to spot her father if he's travelling with a "son". Riiight. But doesn't the old man have to provide identification when he crosses the border? Wouldn't they detain him then anyway? And wouldn't it make more sense for *him* to change his appearance/identity?
Sylvia sacrifices much in order to protect & help her father, including giving him the money her dead mother left her - not that he really appreciates the gesture - just takes it and resumes ordering her around. This guy is so unlikable and WEAK. He lets his kid take care of him and doesn't do a damn thing for her. Throughout the film we see him making bad decisions, and never really taking responsibility - and then the final cop-out (which I won't reveal). Somehow we're meant to sympathize (since Sylvia does), but it's impossible to feel sorry for a silly fool's grand gesture over a cheap floozy who's not attractive or interesting enough to inspire such passion in the first place. It's incomprehensible. Sylvia's character development also suffers, as it's difficult to understand why she loves her father so much when he never cared about *her* welfare. I guess we're supposed to accept the simplistic declaration that her character is Good and Pure of Heart. The true motivations of other characters, like Monkley (Cary Grant), are also fuzzy and unexplained.
Can the acting rise above such bad writing? Not really. Edmund Gwenn is fine as the father, but no fun to watch. Cary Grant does a good job (with little screen time and a role that's shallow, as written) playing a shady Cockney thief - very different from his typical cultured sophisticate role! But I don't think Katharine Hepburn did very well with her dual role. Honestly, she didn't seem terribly convincing as a girl, *or* as a boy! As the latter, she whoops and hollers and runs around, but to me, this makes her seem like a hyperactive child (of either sex), rather than specifically *male*. Ditto at the start of the film when she's still a girl - she's more demure and softspoken, but the high pitch of her voice mostly succeeds in conveying YOUTH, moreso than GENDER. Hepburn's physical acting is also rather superficial - dainty steps as a female (almost a parody of a girl's movements), and galloping leaps as a male. There's no subtlety.
When I hear that a movie flopped at the box office, like "Sylvia Scarlett" did, I want to support it, because I naturally root for the underdog (and besides, so many crappy movies are box office hits). I also thought this film might be ahead of it's time and have something smart to say about gender roles. Well, I found myself disappointed. "Sylvia Scarlett" is NOT an underrated gem. Sometimes, a flop is just a flop. And some films are better off forgotten.