Director Robert Zemeckis said in an interview that the opening shot of a film is crucial to him, and that it rarely changes once it's been designed. Screenwriters could take a lesson from this. Allow yourself some leeway, stylistically, to set the proper tone. I still recall, from my days as a reader, the worst line of dialog to start a screenplay I've ever read. The script, needless to say, went downhill from there.
Proper grammar is crucial.
You need perfect grammar, and you also need to break the rules. Certainly tenses should be consistent, words spelled correctly, and nouns and verbs should agree. On the other hand, sentence fragments are a screenwriter's stock in trade. Anthony peers through the leaves. Three men lie on the ground.