At last it arrested her—and she beheld a striking resemblance of Mr. Darcy, with such a smile over the face, as she remembered to have sometimes seen, when he looked at her.

This scene is so important, but because adaptations turn Darcy into Broody "Byronic Hero" McBroodface, it doesn't hit the right way.

One of Elizabeth's goals in life is to avoid the mistakes of her father. She must know her parents married based on lust; she has watched how it made both of them miserable. She wants to avoid that fate herself. So when Darcy rolls into the parsonage and gives a VERY lust coded proposal, alarm bells must be going off in her head. Darcy likes her out of the blue and he must have her. RED FLAG.

She isn't privy to what we know as readers, he's been struggling against loving her since before Jane was sick at Netherfield. Which means there are scenes at Netherfield when Elizabeth noticed Darcy looking at her with a smile, and thought he was mocking her. He was thinking, "Aw she's cute when she walks" and she's thinking, "He is judging my walking and hardly containing a laugh." This is something which she does not resolve until she sees the portrait at Pemberley.

It's there that she sees the smile that Darcy's family has quite literally preserved for posterity. That is Darcy's real, happy smile, and she recognizes it because he has directed it at her. Which means that him liking her didn't start in Kent, it started long before, which means his feelings are not ephemeral. It is definitely something she assumed, because one of the things she says during the rejection is that he'll get over it quickly.

So we need Darcy to smile at Elizabeth early. It works if it's a very small smile or something that looks like a mocking smirk (there will be no teeth because of portrait guidelines at the time), because Elizabeth doesn't realize it's genuine. And we need Elizabeth to say something like, "He's laughing at me again" when Darcy smiles at her so we know how she interprets it. And then at Pemberley we need that light-bulb moment where it hits Elizabeth that he loved her for a very long time.

This is part of what's wrong with 1995 Darcy. There is no way Elizabeth could recognize his smile, she hasn't seen it yet and neither has the viewer. Even "the look" happens after the portrait scene. The portrait is nothing but a good painting of Colin Firth, not proof of Darcy's feelings. The plot impact is erased. Also, by using a statue in 2005 there is also no possible smile impact because... statue. The scene basically just makes Elizabeth look lustful, which is the opposite of what it should be doing.