Short answer: is it easy or hard for you to imagine the two characters, once having found a private space, suddenly destroying furniture in their efforts to make love?
Long answer:
That’s not a very good example. That particular movie (and maybe her volcano one) was complicated by her whole straight to lesbian (with a side trip to random drugged wandering) back to straight conversion thing. She didn’t have super wonderful chemistry with Ford, but their chemistry wasn’t horrible either, the reports of bad chemistry were IMHO contrived artifacts of the tabloid reports on her private life at the time. So let’s cross that off as a good example.
Let me offer up Meg Ryan and Tom Hanks. Nay, Meg Ryan and pretty much everyone. While she does have pretty good character-to-character chemistry, such as enjoyable banter, you never really get the sense that when these two find some private time that there will be some broken furniture.
Ehhh… it’s quantifiable, but the quantity is in units of “amount of chemistry”. Maybe too circular for you?
Let’s start at the beginning: do you understand, not only intellectually, but also experientially, the concept of romantic/sexual chemistry? Or even other forms of interpersonal chemistry?
I tend to find that it’s a bit fuzzy… I mean, sometimes it’s obvious - you have very similar interests, or you are each other’s preferred physical type, or whatever…
But other times… it exists as a thing unto itself. There can be a person who I would rate as attractive physically and my type, and/or we have similar interests to talk about, or a similar intellectual background, or whatever… but yet, interaction feels awkward. But the there is someone who is outside of your normal physical, emotional, intellectual, or spiritual “type”, or alternatively, there is someone who you have just met, and couldn’t possibly know anything about yet, and still, you feel instantly drawn to them, and you get tingly feelings in their presence and hey maybe you even get a little bit stupid… I mean, how do you explain that? It might be ultimately explainable, but if so, it’s something subliminal and/or subconscious, and therefore not easily explained off the cuff or by a lay person from anecdotal experience.
Back to movies…
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Obviously there is the “real world factor” - whatever the actors are up to is affecting your perception of the characters in the movie. This can be either negative (Ann Heche is really a lesbian or is she?) or positive (Brad left Jenn for Angelina!).
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They do have chemistry, it’s just a different chemistry then the reviewer was expecting. There’s sexual, romantic, emotional, intellectual, conversational, spiritual, origin based, and many other types of chemistry. Although ultimately they all boil down to some kind of attraction or noteworthy compatibility.
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Sometimes, you’re right, it is the script. A movie with a PG13 rating and mostly dialogue and post sex scenes is going to dampen the feeling of sexual chemistry. Also, a very popular trope is to make the eventual lovers transmute conflictual tension into eventual sexual tension (I hate you! No wait really I want your body but was resisting it!) - however, if the conflictual tension overpowers the eventual sexual tension, or overly makes one of the lovers unlikeable, the eventual chemistry can be weakened.
Any questions?