I think the reason Darcy seems so unsmiling and dour to some readers is the POV. Most of P&P is from Elizabeth's perspective. We don't ever get an unbiased view of Darcy. Unfortunately, a film cannot reproduce that limited POV--we see EVERYTHING. For that reason, I think Darcy *is* less serious than movies and fanfic makes him. Realize that *we* are not supposed to be surprised so much when he proposes. In the book, Charlotte mentions Darcy's interaction with Elizabeth. She doesn't explicitly say "hello, he's flirting with you" but the idea that she caught such undertones is implied. Elizabeth immediately rejects the idea, and we readers kind of do too, but on film, it needs to be apparent. Logically, Lady Catherine did not just "hear a rumor" and jump in a carriage. Regardless of what she says, IMO, she had to have seen enough at Rosings to give her an idea that the rumor might be true.
I agree with you. The audience may not be intended to believe that Darcy is going after Elizabeth without reserve, but we should see it. Charlotte saw it, and her view was colored by Elizabeth's opinion. Honestly, I feel as though he proposed rather spur-of-the-moment, but there were still hints that we did not see in the movies because we got the "sad and unsmiling" Darcy.
I agree with you. The audience may not be intended to believe that Darcy is going after Elizabeth without reserve, but we should see it. Charlotte saw it, and her view was colored by Elizabeth's opinion. Honestly, I feel as though he proposed rather spur-of-the-moment, but there were still hints that we did not see in the movies because we got the "sad and unsmiling" Darcy.