Earlier in the Group Read, there was a thread about Darcy's reaction to the Bennets' behaviour at the Netherfield ball, after hearing the rumour of Bingley and Jane's engagement. Chapter 18 says: The expression of [Darcy's] face changed gradually from indignant contempt to a composed and steady gravity. It was suggested that Darcy looked so serious because a) his feelings for Elizabeth were strong enough by this time that they overcame his disdain for her family's behaviour or b)this was how he showed his disapproval, but I think something else was at work.

In ch.58, Elizabeth learns that [Darcy's] resolution of following her from Derbyshire in quest of her sister had been formed before he quitted the inn, and that his gravity and thoughtfulness there had arisen from no other struggles than what such a purpose must comprehend.