The role of the uke (or training partner "receiving" the technique) is the same across most styles. Your job is to provide an attack or target for tori (the one doing the technique) to work with. As the skill and familiarity with the technique grow, your resistance should increase -- but you should always be making them do the technique properly without "cheating" because you know what's coming. I'll focus on defensive applications in the explanation -- but it applies equally to practicing offensive techniques.
Let's say the technique your practicing is a simple step backwards, deflection/strike/block, then full step in with a strike. As uke, when you throw the strike, it should be in range to make contact, and follow the proper principles. (You're practicing that strike, so don't cheat yourself of training by doing some half-hearted punch-like motion that wouldn't scare a mouse.) The first time you're working the technique, you may move slower, you may telegraph the strike a little, and you'll probably leave your hand out there to be deflected/blocked. Tori should not move faster than you! As you practice, you'll increase the speed and focus or force, cut out the telegraph, and be less cooperative about being deflected... Eventually, you should be able to throw different strikes that trigger the same response, attempt to counter, and just plain make it hard for tori to succeed, unless they do the technique properly.
A caution... If you rush through this process, neither of you will be using the technique properly. You'll be using muscle power instead of sound principles, or skipping through the proper body dynamics and positioning. And if you never move beyond the passive level... you're just dancing together.