I have been asked to take pictures at a fashion show and would appreciate some advice as this is the first time I have done this.

I have canon eos 20d and 28-135 lens with 580ex flash.  Should I use high ISO on my camera, what setting is it best to put the flash on.  I normally work on manual on my camera with manual focus but when I went to the dress rehearsal to take some shots, I wasn't quick enough and missed a few of the end of the runway poses.  Would it be better to use automatic in these circumstances or even use no flash and high ISO. 

Hope someone can offer some tips.

Mary

Depends on the lighting! I would have thought there would be a reasonable amount of light - so you can see!

If it were me I would use ISO400-800 - depending on the lighting and the metering that gives - and if its still too dark, then use the camera in manual and let the flash do the correct exposure - if its not too dark use Tv and say 1/100 and use the flash as fill - I would also use FEC (Flash exposure Compensation) depending on the histogram - I shoot raw and make sure the histogram is to the right, but not blowing out the important stuff.