I have a manuscript for a play that is entirely left formatted. I have created styles for each element to format them to "standard practices." Is there any way to select all the instances of a word (like a name) and apply the style across the entire document? What I have to do now is manually select each instance to apply the style. (CTRL/CLICK works, but I have hundreds to change.)

You can change styles using Find/Replace.
In the Replace dialog, click on the More button and then format>Style