Uke's role in training honestly; committed attack, don't give up unless you have to.
Nage's role in training honestly: make the technique work, make uke submit.
It is fun to hear someone who outranks you by several degrees say something like, " You can't do that, out on the street an attacker won't react that way."
My response, " I just did and if I was the attacker I would."
Aikido training in the dojo is not quite the same as training for external conflict.Basic techniques in the dojo are used as conditioning exercises.By repetitive movements you acquire an aikido body.In the street your potential assailant has generally no idea of issues like blending.Their responses are not like the response one gets from a experienced aikidoka.The responses from an outside attack is more like training with a strong , awkward gent who wants do you a bit of damage.Not quite the same as mutual training in a dojo with a buddy.