I am posting this for reference.

(1.24) Questing Successfully

Questing successfully and the physical placement of progress tokens are two separate game occurrences that happen in sequence during the Quest Resolution step. As soon as the players determine that the total committed Willpower is greater than the total Threat in the staging area, they are considered to have quested successfully. Any Forced or passive effects initiated by questing successfully resolve before physically placing progress tokens.

Question: The last sentence of that entry in the FAQ omits Responses. Does Mirkwood Explorer's Response trigger before or after progress placement?

Answer:

Since the Explorer’s Response is tied to questing successfully (and not the placing of progress), then it would resolve before progress is placed.
Cheers,
Caleb