Do quest items and enemies appear only after I've taken the related quest or they are presented on the map from the start and I can grab the item/kill the boss first and receive the quest reward later?

Edited December 10, 20196 yr by Hawke64

There are some NPCs and items that only appear if you agree to do a quest, but in many cases you can find NPCs or quest items before being offered or agreeing to a quest. For example (unimportant early area spoiler alert, but just a warning in case you don't want spoilers):

Spoiler

In the Emerald Vale, in order to find and kill the marauders in the wanted posters, you need to agree to offer to hunt them for Constable Reyes. However most of Ludwig's errands can be completed before you talk to him.

However I can think of several more quests throughout the game where NPCs, items, or areas do not appear until you agree to a quest. Sometimes they are justified (most of the area gates are mostly understandable), sometimes they are not.