To me that "I don't know what you think he's like," was SO great because it was uptight and defensive and nervous and it was so obvious he didn't like her poking around in there and more, wasn't really prepared for it. Like they've made some teasing comments before such as the, "Congratulations," at the fake wedding, but this is really delving into it and the specifics in a way they just don't do usually. Like even in the kitchen scene back in "A Little Night Music" he asks, "Are you in the middle with Brad?" which is requesting no more information than she needs to give him. And some of that is probably due to how many times they've run a honey trap... it's just a routine conversation by this point, but there's definite tension in the way Elizabeth keeps flirtatiously pressing Philip and he's sighing and dropping his hands with a thump.