Do they ever choose to act foolishly on purpose, and if so, why?
Like we've stated before to your question, it depends on which character you are referring to. The reason for this is because Jane Austen wrote with various purposes: First, she wanted to entertain. Second, she wanted to inform the reader of the insanity of her era: How people lived up to appearances, how they seemed to change personalities, how they switched their main interests in life, and how tradition ruined any chance for progress in people's life.
All of the most colorful characters are the ones who did not change, say, for example, Sir William and the rest of Anne's sisters. This, however, is a technique to reflect the thoughts of the time that the aristocracy could not change (or else) they would basically give in to the growing bourgeoisie. That would have been an embarrassment to the Elliots, and this is why Austen brought this up.