I try to be a good uke in the sense that I try to give a good an honest attack. I only interupt the technique at very rare situations where an opening is very obvious. I try to adapt my attack to what the nage wish to practice - some people like me to come charging in with all of my 190 pounds and some people like me to just slightly irritate their sense of perimiter in a slow attack. It very much depends on what the nage has as a focus in that particular training session. I also try not move somewhere by myself to 'help' the nage, but on the other hand I try not to be too resisting. I guess I try to keep moving for a better position all the time - only pausing if the nage really needs to take a second to think.
This is what I try to do when doing ukemi - It's not up to me to decide if this is what I actually do