Wizkids Rules!

Q: Please explain the interaction/timing of “When Fielded” and “While Active” effects.
A: Characters are fielded when they are moved from the reserve pool to the field zone (by paying their fielding cost or through any special effect). An effect that occurs while the character is “active” is described as an effect that takes place when one or more of that card’s dice are in the field. See the Tournament Rules - Active and Fielded section in the rulebook on page 19. For example, Hawkeye Longbow’s effect would activate only when you pay the character’s fielding cost and move the die from your reserve pool to the field. Doctor Doom Reed Richards’ Rival’s effect would stay in effect as long as that character die is in your field zone.
For “When Fielded” effects, you ask “Was the character fielded?”— If “yes”, the effect occurs immediately as they are fielded. This will happen simultaneously with any static effects already in the field (like Doctor Doom Reed Richards’ Rival’s -1A/-1D). So, if I field Hawkeye on his Level 1 or 2 face (with 1 Defense), if there are no other defense modifiers on the field, it would play out as:
1) Hawkeye deals his attack value (which is being modified Dr. Doom RRR by -1) to target character.
2) Dr. Doom’s RRR modifies Hawkeye’s Defense by -1 KO’ing Hawkeye.
So, effectively, Hawkeye uses his effect just as he is KO'd.