Twelfth Night starring Toby Stephens, the effervescent Imogen Stubbs, and Helena Bonham Carter at her comedic best particularly does a fantastic job in establishing the lead characters' connections.
Toby Stephens is, to put it mildly, a sexy piece of compact masculinity. In other versions, Orsino is tall-almost-stooping, melancholy and extremely self-pitying. I always wonder what on earth an adventurous, sharp and funny chick like Viola sees in the guy!
Toby
Stephens, however, plays a tautly wound chap who is normally extremely
energetic (and witty). He does not so much swoon with love as
glower with it, and viewers get the impression, as they should, that his
problem is offended pride more than an offended heart. He doesn't
really care about Olivia; he's just bored and she's available and he
thinks she should like him. He does care about his servant Cesario (in
fact, Stephens' reaction as Orsino to Cesario's "betrayal" is gripping
and indicates what really matters to him); he is overwhelmingly relieved
when Cesario turns out to be female.
Olivia and Sebastian also come across as believable. For one, it seems to be based on Olivia being taken with a pretty boy who chastises her. Olivia is arguably simply lighting on the first person who ISN'T Orsino to fall in love with.
Still--at the end, one can only hope that Sebastian is an adequate substitute for, uh, his sister.
Shakespeare being Shakespeare, one suspects that the problem of substitution IS Shakespeare's point.