Nothing to do with grammar.
Maybe Spanish is different in this regard, but spoken language rarely sticks to all the rules all the time. Need examples of things which happen in conversation but buck the style conventions of proper written English? Sentence fragments. Verbal, verbal restarts. Sentences that simply stop halfway-- you get the idea.
Writers, whether they work on screenplays or novels, write dialogue how people would speak in real life. Grammatically correct English would immediately make a character come across as fake, unless the character is a school teacher from the year 1831.
Oh! Another difference between written and spoken English. Given linear time, a speaker can't go back and correct earlier sentences. So dialogue tends to be non-linear.