Can items/characters from the big box expansions such as Island of Dread, Midnight, etc., be used when playing the base game? (and vice versa)

(or are components from the Runebound base game, and expansion card sets, not interchangeable with the big box sets?)

The characters from the various expansions are pretty well balanced, and you can use the new ones with the base game. The Isle of Dread characters are maybe slightly stronger than the others, but it's not a large difference.

There are a few item/ally cards in Isle of Dread which you can use with the base game, but for the most part the big box expansions don't add more item cards. Well, there are a bunch of items in Midnight, but I've read that they don't work as well with the normal game. (I haven't played Midnight, myself.)

The characters can be used in the base game, although keep in mind the Midnight characters are a bit underpowered compared to those in other expansions and some of the Sands characters have a skill bonus that is sueless outside that expansion (Prophecy).

Usually, the new items/encounters in the expansions are not useable in the base game.

I've actually had no issues mixing characters across boxes; outside of the characters that would have been a bit under my groups play style/taste any way. And as far as I can tell, there are absolutely no issues with making the mega market stack either. Challenges though; well, those weren't supposed to be mixed, but if your creative enough, why not.