Dialogue blocks begin with an uppercase CHARACTER NAME, followed by the dialogue. You can also press tab to begin writing dialogue, with auto-completion menu for the character names in your screenplay.
Beat automatically recognizes parentheses inside a dialogue block, and you can have any number of them inside the block just as easily. If you want to add parenthetical lines inside an existing dialogue block, press ⇧shift–↵enter to avoid double-line breaks after dialogue.
If you are wondering why your FADE IN (or anything in all caps on a single line) is formatted as a character cue, don’t worry. Keep on smashing ↩enter, and after two line breaks Beat will figure out that it’s not a character. See this example from Big Fish by John August:
Dual Dialogue
Due to the linear and plain-text nature of Fountain screenplay format, writing dual dialogue requires a small trick. Type in ^ after the second character cue you want to appear alongside the preceding dialogue block.
This might seem a bit weird and could require you to constantly check your print preview at first. Just remember that special character (^). On macOS, you can also press Format → Dual Dialogue or press ⌘cmd+⇧shift+2 anywhere in the second block to make it dual dialogue.
Check out the Tutorial for a working example.