Sibilants -- "s", "z", "sh" sounds -- present a problem for many fiction writers. Why? Because beginning writers in particular often don't pay attention to whether or not sibilants are actually present in dialog.

Read this and see if you can spot what's wrong:

"Get out now!" he hissed.

Figured it out yet?

In order for someone to "hiss" something when they speak, there has to be at least one sibilant present.

Too many writers are doing this (I spat), and it's seriously got to stop (I hissed).

"Stop saying such things!" the noders hissed. That works because there is a sibilant present. Otherwise, it ain't hissing, folks.

(For that matter, dialog qualifiers like "hissed", "barked", "spat", "yelped", etc. should be used only sparingly, or else you run the risk of making your characters sound like a bunch of barnyard animals).