Even if you want to describe the characters (and the sooner you do it, the better-- like celina said, you don't want to have the readers making up an image of a short, thin brunette and then shatter that on page 123 that she is actually tall, blond and plump), there are many ways to do it without being obtrusive or stopping the movement. Make the descriptions part of the scene and plot, too.
Again, in P&P, not much description until the dance hall scene. And only a few choice words, we have images that Jane is 3 times as beautiful as any other girls there, Lizzy is handsome but not handsome enough (for Darcy), Bingley is good-looking and amiable (but Mrs. Bringley said he was extremely handsome, obviously jaded), and Darcy is tall and proud, with an air of superiority, and draws all the attention to him even though he doesn't want it. Again, only a few words (although, of course, back in Austen's day, it's more "tell" than "show" but still, you get the idea). And much of it is about how other people REACT to these characters.
The great thing about it is that we can have very different ideas of the characters based on our own backgrounds, cultures, and times. People in 1909 may have a very different image of Jane or Lizzy than people in 2009. And yet, the scene still works perfectly in the context of the story. In fact, I doubt that a laundry list of attributes (hair, eyes, body types, etc.) would have better impact than just a few choice observations.
Not to mention it allows the casting director of your movie a lot of freedom.
