Not only romantic scenes, but any emotion. We are watching actors, any emotion on display is some measure of fake depending on how good the actor is.
That said, we all grew up watching movies, and one could argue their way of expressing emotions are imprinted onto us, so we all ‘emote’ as actors, and their emotional acting matches our true emotions.
You could expand this argument to our sexual realms, and it explains why young people imitate or expect it to be like porn. I wonder if also our romantic/sentimental emotional range tends toward the dramatic because of this effect.
In short: media shape how people feel and behave, more than the other way around.
If you actually act a realistic way then the (general) audience will completely miss it.
You have to drop your jaws, screech, and other over the top expressions so that everybody understands what emotion is being conveyed. A quick smirk when the villain deceives the main character will get missed; it needs to be a mischievous chuckle that clues everybody (but the main character) in on the fact somebody has been duped.
Good actors do bring their own emotion in. (At least some do)
They might not feel anything about their acting partner, but they feel about other persons and bring that emotion into the play, to make it more real.
Similar to how the best lies are those that contain as much as truth as possible.
I know that I cannot watch anything with mechanical acting that is obviously fake (probably the majority of productions).
Speaking of it, Asteroid City is a very good movie that explores this theme.
By the time Robin Williams filmed Good Will Hunting, he'd already dealt with his own drug issues and been struggling with sobriety. Hell, he visited John Belushi a few hours before Belushi overdosed.^
The article doesn't mention it, but the park scene is a response to the earlier office scene.
Office scene (aka painting scene): https://m.youtube.com/watch?v=SuBx0oy__EI
Park scene: https://m.youtube.com/watch?v=8GY3sO47YYo
Full disclosure: outside of Casablanca, these two scenes are my favorite in cinema
One of the things that makes them so watchable is the little ticks that Robin Williams puts in (probably from his improv experience). Maybe he doesn't have the precise lived experience, but the man knows how to emote trauma.
In the office scene, there's a neat turn right before the painting moment where they're talking about weightlifting and Robin Williams' character gives a "Don't fuck with me, kid" warning response ("285. What do you bench?"). Of course, Matt Damon's character just zooms right past it.&
Similarly, the park scene has a great beat where after he says "You've never been out of Boston." he waits for Matt Damon to acknowledge what he said.+
^ https://web.archive.org/web/20140816025724/http://franksreel...
& Office scene, 2:03 https://m.youtube.com/watch?v=SuBx0oy__EI&t=2m3s
+ Park scene, 0:41 https://m.youtube.com/watch?v=8GY3sO47YYo&t=41s