I knew that Charlotte encouraged Mr Collins after his failed proposal to Lizzy, but I'd never noticed before how very quickly she put her plan into action.

The very day Mr Collins is rejected, Mrs Bennet sends everyone out of the room, so she can talk to him - all leave but Lydia "determined to hear all she could" and Charlotte, "detained first by the civility of Mr. Collins … and then by a little curiosity, satisfied herself with walking to the window and pretending not to hear." (ch. 20)

For the rest of the day, "the assiduous attentions which he had been so sensible of himself were transferred for the rest of the day to Miss Lucas, whose civility in listening to him, was a seasonable relief to them all, and especially to her friend." (ch 21)

But this wasn't done just to spare her friend, of course. The Bennets dine at Lucas Lodge the next day where we learn that Charlotte's "object was nothing else than to secure her [Elizabeth] from any return of Mr. Collins's addresses, by engaging them towards herself." (ch 22) And Mr Collins proposes the next day. He "hasten[ed] to Lucas Lodge to throw himself at her feet."(ch 22)