During the scene in Ch. 13 when Colonel Brandon's letter arrives and he must immediately depart for London, Mrs. Jennings is doing everything she can to get him to spill his secret--whatever it might be.

Sir John, who, as we have noted, really only knows superficial things about Willoughby and little or nothing about his character or personality (other than that he's fun at parties), says two interesting things about his friend here:

"There is no persuading you to change your mind, Brandon, I know of old," said Sir John, "when once you are determined on anything."

Colonel Brandon is always mindful of others' feelings and is always polite and gentlemanly, yet what Sir John says here sounds like real stubborness. Or perseverance? Does this not seem far beyond the scope of what Sir John typically perceives about other people?

The second thing is a reply to Mrs. Jennings request to Sir John to find out what Colonel Brandon's business is. He answers: " "I do not want to pry into other men's concerns. I suppose it is something he is ashamed of."

Does that not seem like an extraordinary (and possibly out of character) thing for Sir John to say in company? To say that his friend might be leaving because he might be ashamed of something?