Writers tend to be more sensitive to spelling and grammar issues than anyone else. If you hold writing sacred, you’ll likely be protective over how it’s used. With modern technology, it’s become less of an imperative to know when you’re making a spelling or grammar mistake because a red or green line will just show up underneath the words to alert you. In screenwriting, you don’t always have to get rid of those lines (though you should ALWAYS use the check before you submit anything.)
There are many different kinds of writing. For each style and genre there are different rules. In a scholarly journal, everything should be formal and grammatically correct. In free-form poetry, you can do whatever you want. In a blog, you can basically get away with anything as long as it is decipherable (and you have a forgiving audience…) Screenwriting is somewhere in between all of these. There are rules and guidelines that you should follow but you can also exercise freedom in certain cases.
The general rule of screenwriting when it comes to spelling and grammar is that you should only break the rules if it contributes to the story or character. If you write “your” when you mean to write “you’re” then (not “than”) you’re making a mistake and you need to fix it. The same goes for referring to the same character as Rachel and Rachael. However, if proper grammar would be unfaithful to the character (or your narrative voice in scene direction) then you should break the rules.
Some of your characters wouldn’t know the difference between “who” and “whom.” Even people who do know the difference wouldn’t necessarily think about which one to use in conversation. Everyday conversation doesn’t always follow the rules of grammar, so don’t worry too much about it in your dialogue. In fact, incorrect grammar might make your characters more believable. A wild-west outlaw wouldn’t use the same grammar as an English nobleman or a 12-year old beauty queen. Make your use of grammar in dialogue specific to your characters and not to the green line that appears underneath the words.
The scene direction can also contain some grammar issues, as long as it contributes to your narrative voice. You should avoid any glaring mistakes that would enrage a reader but you can usually get away with sounding somewhat conversational in the scene directions, if you’re consistent with it. The scene directions should be enjoyable to read and describe the action in the best and most efficient way possible. If you have to end a sentence with a preposition or split an infinitive to do that, it probably wouldn’t hurt all that much.
In summary: always use grammar and spell check. Always. Just know that sometimes you are allowed to click “Ignore” and know that you’re doing the right thing. And forgive bloggers for starting sentences with “and” and being liberal with punctuation.