You can emphasize using the pitch function. One way to do it would be to add the script to the timeline in segments. Where in some segments you can have different a speed and pitch to produce emphasis.  

For example, if you used this sentence - "We were walking at the park then suddenly heard someone say, "WATCH OUT!"

You would type "We were walking at the park then suddenly heard someone say, " in your regular voice then add it to the timeline.

Once done, clear the text field then type "WATCH OUT!" Change the speed and pitch or even change out the voice completely to something that sounds alarming. Add it to the timeline and it'll be played continuously. From there, just keep adding more of what you want to be said.

Additional customizations such as delays and distortions are on the way with future updates.