Footsteps clatter loudly and significantly. Characters gesticulate and argue behind glass, seen but unheard. Pumps pump menacingly. Characters breathe loudly, and if you know Jacques Tati, you know where the movie makers got their idea for the hospital lobby scene with its busy ambient sound and utter absence of dialogue.
A lot of this stuff comes across today like '60s artiness, but it works better than do the period touches in other '60s movies I've written about here. For one thing, the filmmakers took music more seriously and used it a good deal more effectively than did the makers of Harper. And they did just about everything better than did the folks who turned Modesty Blaise into an unfunny proto-Laugh-In sketch in 1966.
All right, folks, what defines period style for you, whether in movies from the 1960s or from any other period? If you behave and answer this question, I'll let you watch Bullitt's famous car chase. (One feature of the scene's soundtrack struck me as odd, another as interesting. I'll be interested to see if anyone else notices them.)