Favorite: Adding Notes! I really love adding in the different parts to highlight dialogue, chapter changes, the little notes, all of it! Especially the notes -- it really helps to get my creative engine running and get me on a roll with writing scripts. When I do author notes, I have a clear path on where I'm taking the story and if I have to come back to it later, I know exactly where I left off!
Least Favorite: When I have to edit for repeated words. A lot of times, I just write what comes to me in the moment, and usually I can use the "Find and Replace" option in Word. BUT, when it says I can't search because I used the word too many times for it to be pulled up, that's when I have to do it all manually, and if I've done more than 50 pages of script, it's a nightmare XD