Are the speeches in Acts just made up by the author?

No - though answering it well needs one distinction. Ancient historians did compose the speeches in their works; Thucydides, who set the pattern, openly said he gave speakers the words the occasion seemed to call for. So Luke, like every historian of his age, has shaped these speeches into his own Greek. The real question is not whether he polished them but whether he invented their substance - and the evidence says he did not.

Thucydides' own rule shows why the convention was not a licence to fabricate: the historian was also bound to keep as close as possible to what was actually said. Where we can test Luke, he honours that. Set his Gospel beside Mark, the source we still possess, and the surprise is how little Luke invented, not how much he changed. The speeches in Acts likewise keep details an inventor would have smoothed away: the earliest carry rough, un-Hellenised phrasing that points back to an Aramaic substratum, and Paul's address at Athens is quietly steeped in local colour, quoting back to them the Greek poets they claimed as their own (Acts 17:28). That is the texture of careful reporting, not of free composition.

None of this claims the speeches are word-for-word transcripts - no ancient writer worked with a recorder, and the point is the reliability of their substance, not exactness to the syllable. It shows them to be faithful reportage in the idiom of the day, not the author's fictions. For the wider case that Acts holds up as history, see our deep dive on the archaeology of the New Testament.