When writing dialogue, it’s tempting to throw in a lot of stammers and hesitations to mimic how people talk in real life. You, um, really shouldn't, like, do that. Instead, just write the meaningful words that your characters must say in order to get their point across and advance the plot. This means that you should avoid using words such as “uh,” “um,” “well,” “so,” “er,” “like,” “anyway,” etc., in your dialogue because they just bog down your script with unnecessary filler. If your dialogue contains only the words that matter, the actors will fill them with life and emotion, including their own stammers and affectations. Imagine if someone were trying to write a conversation between Hugh Grant and Denholm Elliott in the way that they would actually deliver the lines. That one scene could fill the entire second act of your script. Just write the important words and leave that extra business to the actors.
Of course, the same exception applies to this rule as applies to every other rule in screenwriting: if it makes your story better, then put it in your script. If you need to use specific leading words in order to relay a character attribute that's important to your story, then you absolutely should do that. For example, if a character always pauses to think when someone asks him a question by saying, “Yes, I see,” before replying, then that’s not only a potentially interesting way to relay that information, but it’s also a fun character quirk for an actor to play around with. However, if you don’t have a specific reason like this for including meaningless words in your dialogue, then they don’t belong in your script.
