There are a lot of modern misconceptions about the nature of the content of ancient texts. For instance, I’ve seen depictions of St. Paul or a Gospel writer hunched over a table writing by candlelight. That’s almost certainly not how the texts were developed. In fact, Luke/Acts are probably the only two New Testament texts that may have been directly penned by their author.
St. Luke was highly educated in philosophy, medicine, and the arts. In addition to working as a practicing physician, it appears that he also may have worked as a scribe. He may even have served as St. Paul’s scribe for some of his letters. (I’m using scribe in its more common ancient context and not in its specific first century Jewish connotation, which is rather different.) So it’s virtually certain that he directly penned his Gospel and the Acts of the Apostles.
While it’s very likely that Paul, the other epistle writers, and the writers of the Gospels other than Luke spoke Greek, the lingua franca of the Empire, with varying degrees of proficiency (and as a Roman citizen, Paul at least almost certainly spoke some Latin), their native language was Aramaic. While Paul and possibly some of the others had training in rhetoric, it’s unlikely they had training in the more specific craft of capturing that rhetoric in written form. We know that Paul used a scribe for his letters because he mentions that fact in some of them. It’s safe to assume the other authors did as well.
It’s important to understand that an ancient scribe was not like a more modern secretary taking dictation or shorthand — trying to capture what is said word for word. Rather, the job of a scribe was to find the best way to convey the thought and intent of the speaker in written form. In the case of the epistles and three of the Gospels, the scribe was likely also translating from the author’s native Aramaic to Greek. Even when not translating, the scribe was often responsible for choosing the best written words to communicate the desired thought. The act of composing a text was more of a synergy between the author and the scribe than a mechanical reproduction. (Apart from the fact that I don’t believe we have sufficient text to determine authorship from textual analysis alone for any of the New Testament texts, I also think it’s a futile quest for this reason. The same author working with a different scribe would produce a text with a somewhat different “voice.”)
The author and the scribe would work together to produce a text and then the text would be sent with someone who had been trained to properly deliver it to its recipients. Nobody who carried a letter or other text of any complexity in the ancient world was a mere delivery agent. They weren’t the ancient version of UPS or FedEx. Rather, they were the ones entrusted with the task of correctly presenting it orally. So it’s important to recognize, for instance, the true role of the deacon Phoebe carrying the letter to the Romans. She is the one who would have stood before the Christians in Rome and orally traditioned Paul’s teaching to them. It was a very important and even crucial role.
When discussing the texts of the Holy Scriptures, I find many people tend to make pretty anachronistic assumptions about the way they were composed. Hopefully this clarifies some of that confusion.