Secondary character who was more intriguing to you than the main romantic interest?

That woman in CHARLIE CHAN IN PARIS talked about on here before (so I checked it out to see for myself).

Dina Meyer in STARSHIP TROOPERS (though I think that was intentional to play games with viewer expectations).

It would be tough to be director on a movie where you know someone else should be the lead but you can't override the decision.
I've seen a few movies where the most inexplicably bland people get important roles--you just know it must be some kind of nepotism. ED WOOD made jokes about it, but they had amateur outside investors dictating the casting, when that sort of thing happens all the time inside Hollywood too.

There's a story I heard about JAWS--that Linda Harrison was the alternate consideration for Brody's wife (due to her studio executive husband). In that case Lorraine Gary was a much better choice (she also being a studio wife).

I remember seeing various comments in regards to Laura Fraser’s blacksmith character, Kate, in the movie A Knight’s Tale (2001) and how Heath Ledger's character, William Thatcher, should've shown an interest in her rather than Shannyn Sossamon's Jocelyn character (who he seemed to stalk/be obsessed with). I have to say...I agree with them.