different partners in training
As the tree like guy of the dojo, I sometimes get intrepidation at either seminars or with new people in the dojo. There is always the fear of not being able to move the guy whose arms are like some people's legs ... but it goes away after they throw me, correctly, in five or six months. (JUST kidding, I play nice)
Establishing your posture, sometimes rooting, leading your partner, and executing a technique can be learned mechanically, or you can blend the circular/balance version of taking away the balance and pulling into your sphere of power, it doesn't matter. You get a moving body that is directed by YOU to YOUR advantage.
A lot of time can be spent starting the movement, following the movement, and leading the movement ... but that is usually the crux of practice, using figure eight movement to continue motion, or interpreting the weapons or sword movements that translate into hand to hand techniques. You can not learn the thousands of variations by directing one body, one size, one set of responses ... that is the mechanical theorey, not the actual real world.
If you watch a practice, you will see little variations of each practitioner, no two people are exactly alike. That is the way of nature. Even if they look alike, and are within a set of guidelines, deviations and variations are inevitable? So you might as well bite the bullet, accept the inevitable ... jump right in and experience the humiliation of stumbling, falling, looking stupid until the natural effect of practice gives you the skills you need to move anyone, throw anyone, and stay ahead of even the most fearsome ukemi. (HEY, you were brave enough to write a question on the Aikiweb.)
It took a year for the guys/gals of my dojo to realize I was merely using restained force to effect a correct technique, not trying to rip off their arms and break their bodies? (Although I am much more gentle in my old age.)
If you don't go with everyone in your dojo, a seminar will be just plain terrifying, and seminars are about the most fun you can have in Aikido practice! Try it! Give it some time, it will come to be.