Well, let me start off be saying that John Williams is Da Bomb! I mean, who is better than JW in today's society? That's right, no one. So, I have chosen to remark on his music from (you know it, you love it) Jurassic Park (I know it's not classical, but, hey, that wasn't in the criteria...). However, in all actuality, I only chose this piece because it was within reach of my desk chair.
I'm a huge Spielberg (and Spiegelberg) fan as well. I have 4 of his autographs (I'm not so sure about two of them...) just for kicks.
Let's get down to it.
There is only one mixture chord and it occurs 4 times according to my own count. It is a bVII and always occurs before a V chord. During the first three occurences the progression is I-bVII-V-I. In these cases, the chord is a 9 chord in 2nd inversion. However, the last occurence is I-V-bVII-V-I. In this case, the chord is simply a root position triad.
The desired affect for this chord is most likely tension. This can be felt by both the performer and the listener. However, once the performer practices and listens to it more than once, the surprise of the tension goes away and they are left with a colorful chord progression to elevate the piece. The same goes for the listener. It's like watching a movie! Of course, if you've seen the movie a dozen times or more (like me...) then you know what is going to happen. Besides all that, the surprise of the tense bVII chord when it is heard for the first time, and even the second time, is encouraging to our musical minds.