Applauding between movments
There are lots of people that think you shouldn't - but I'm not one of them.
and to support this opinion is a snippet of a conversation between Hilary Hahn and conductor Eiji Oue (found on HilaryHahn.com)
"Q: Applause between movements (sections of a piece) in a concert?
A: Well, I think it's great. You know, if the audience is genuinely excited, and applause breaks out, that's good. I figure that if genuine emotion leads to applause, then why not.
There are a couple of stories to illustrate this:
A few years ago, a conductor performed Beethoven's 7th Symphony, and the audience was very enthusiastic. They applauded heartily after the first movement, but he was so upset by it that he turned around and stopped them. After the second movement, there was a small spattering of applause; he turned around and stopped them again. After the third movement, the audience was very tentative. And finally, after the great, rousing ending of the last movement, at the end of the symphony, nobody applauded once they lost the opportunity to start clapping, they couldn't get back into it. The conductor had to walk offstage to silence.
Mozart, on the other hand, wrote a letter to his father after a performance of his Paris Symphony. He reported proudly that, after the second theme of the first movement, people applauded and he was so excited. If it's good enough for Mozart, it's definitely good enough for me."
In another interview between Hilary Hahn and Grant Cooper of the West Virginia Symphony.
"GC: I actually think that there are certain pieces (such as the end of the first movement of Tchaikovsky's first piano concerto) in which it's very difficult for anyone wrapped up in the music not to feel that they've come to the end of a major musical statement, especially if it's been performed in a way that communicates with them. There's no question in my mind that an applauding audience's enthusiasm is genuine. In fact, between-movement applause used to be part of concert etiquette. When I have the chance, I sometimes tell the audience that nothing will happen if they clap between movements no thunderbolt will strike them down.
I've never feel any degree of disappointment when the audience applauds before the piece is over. I'm just glad they're there, and it's my proof that the audience is awake, that they're alive, thinking, and enjoying. Enjoyment is good."
Although someone told me the time between movements should be kept quiet to allow the musicians that time to focus on the next piece - and I suppose some performers need this. Personally, I find the coughing and random noises from an audiences that's been supressing them for the last however long to be much more disturbing than applause.
So... if you ever get the chance to hear my music, and you feel lead to applaud at the end of a movement, please, let the performers know you appreciate what they're doing.