Can Defend Your Honor interrupt Challenge On The Field.

I thought no but someone told me differently

Yes it can.

Defend Your Honor interrupts step 6, so its duel would take place after the participants are chosen for Challenge on the Fields, but before doing anything else. Defend Your Honor can also interrupt another Defend Your Honor (though both duels would likely end up with the same winner).

So choosing participants is considered an effect?

Huh

Ok thanx

Choosing participants is not an effect. Defend your honor triggers before effects occur, but after costs are paid and targets are chosen.