Eric Heisserer offers a good example of why screenwriters need to read dialogue aloud:

“Mos-top eeple ike ash.”

It sounds awful and hard to understand. The other choice is to slow down and enunciate each word. Also awkward.

Reading it on the page, you wouldn’t think it’s such a problematic line of dialogue:

DEALER

Most top people like cash.

In this case, it’s the duplicated consonants that make it confusing. But there are many reasons a line can look fine and sound terrible, including repeated sounds (“win in Indiana”) and homonyms (“violent sects”).

The only way you’ll know is to read dialogue aloud as you’re writing it, and again with fresh eyes.

Dialogue needs to fit both the moment and the mouth. I’ve found actors can sometimes finesse a line that would leave me tongue-tied. But it’s rarely a gamble you’ll want to take, particularly if you’re not going to be on-set.