Another entry in the category of micromanaging how actors will read their lines, using typeface embellishments like this in dialogue will only detract from your story and –yes– piss off your actors. Neither readers nor actors want to read a script that tells them which words to emphasize in their dialogue. It slows down the action and comes across as just another attempt to direct from the keyboard. Which one of these looks better:
“How dare you speak to your own mother that way, LITTLE BOY?!”
“How dare you speak to your own mother that way, little boy?”
Is there really any reason to include the typeface embellishments in that first sentence? I even took out the exclamation point, since that also tells people how to read the line. Read the second, unembellished version of that line to see how many different ways you would naturally inflect those words. There are only a handful of ways to do it and all of them work fine. If you use bolds, Italics, etc., in your dialogue, then you are only limiting the range of choices that an actor has, which is something that you never want to do.
I also frequently see people use this kind of font embellishment to emphasize words in action descriptions. This also reeks of amateurism. There are only a handful of times when you want to use anything other than plain 12-point Courier. If you want to make sure that the reader doesn’t miss a certain prop, then underline it. You can put sound cues or effects in CAPS, but don't overdo this. People usually write foreign words in Italics, which is fine as long as it isn’t overdone to the point of distraction. There is never a good reason to use bold type anywhere in your script. The best rule to follow is to avoid using any of these typeface embellishments unless absolutely necessary.