Dialogue is self-explanatory but there are many things you should not do when writing dialogue. You should not use exclamation points or underline or bold for emphasis. You are not directing this movie and you cannot direct from your typewriter. Leave that to the actors and the director. They will only be annoyed at your interference in their job and, if they get over that enough to read your script, will ignore your emphasis anyway. Besides, you have no way of knowing what the right emphasis will be or what works until you see it performed. Even if you plan on directing, you should leave this out so you can negotiate these things with your actors first instead of tainting their interpretation of the lines.
You should not use parentheticals for same reason. Parentheticals are notations below the character name when writing dialogue, like:
That’s my cup of tea.
Again, do not direct from your typewriter. The actor playing Andy should know how his character would say the line. He should be able to infer that from reading your script and working with the other actors and the director. That’s not your job. Besides, how does one act annoyed? That is bad directing anyway, you should never tell an actor to be an emotion. Nonsense.
The time you should use parentheticals is when the words are at odds with the context of the script. Like so:
Go to hell.
Also, please avoid meaningless lines like:
Andy looks away.
Andy takes a drink.
You should only write these lines if, on the first example, Andy looks away as Peter puts poison in his drink. The second example would then be critical and necessary if Andy were to take the poison. Many times, people throw these lines into the mix because they are trying to give the actors something to do on long, boring tracts of dialogue. Let the director and actors figure out when they look, where they look, what they are doing in the scene.
Please have a look at the sample screenplay attached for an example of proper short film screenplay formatting.