In my experience, there'll be less likely to recieve fanmail during previews, therefore when they get your letter they may reply more quickly (due to less amount of mail). However, constant rehearsal process also means that they've got little time on their hands during the day, so arent at liberty to reply at the time...

It really depends on your choice. Either way, your letter will get to the actor just the same as if it was during the proper run. When they get it and when they reply.... well we can only guess

When compared to those who send after opening night, I would say there would be less of a chance that your letter would reach the actor if you sent during previews. Why? There aren't as many shows during the week during previews, meaning the actors aren't at the theatre as much to answer fan mail. I recommend waiting a couple weeks and send your request after opening night.

Khackdrums

If I were to send something to a play thats previewing right now and doesnt open for a couple of weeks, would it still get to the actor?