Romans: Is that you speaking Paul?
Recent scholars on Romans has identified several instances of Speech-In-Character, where the a character other than Paul is speaking. I agree with Witherington's assessment that this is because Paul was writing to a church over which he had no direct authority, and therefore needed to be a bit indirect in his arguments.
It is fairly universally accepted that Romans 7 contains a lengthy monologue by a character who is not Paul who speaks of their struggles with sin and the law. It has also long been noted that Paul often asks rhetorical questions in Romans. Scholars seem to widely agree now that these are mostly not rhetorical questions, but rather indicate a dialogue between two characters.
To give an idea of how endemic this is in Romans:
Romans 1:18-2:16 is sometimes regarded as a dialogue between a gentile moral preacher and Paul. Much of Romans 2:17-4:25 is generally accepted to be a dialogue between Paul and a Jewish teacher of the Law. Romans 7 contains a long monologue by a non-Paul character. Most ancient commentators thought most of Romans 9 was a Jew who was not Paul speaking.
In some places in Romans there is some substantial level of difficulty involved in identifying who the characters are and which one is asking the questions and which one is giving the answers. eg Stowers argues that in Rom 3:1-8 Paul is the one asking questions and the teacher is giving answers. Campbell thinks most of 1:18-2:16 is a Jewish preacher speaking. I think the strength of the parallels between 1:18-32 and the Jewish work Wisdom of Solomon mean that it's the Jewish teacher there, but contrary to Campbell I think the voice changes to Paul in 2:1.
To the original intended audience it would have been clear what was going on because Phoebe whom Paul sent with the letter would have read and presented it in such a way as to make it clear (changing voice, expression, body movements) as she read / presented / acted out the letter. Whereas we do not have that luxury.
Of course this creates substantial difficulties for us in trying to understand the letter. It certainly makes difficulties if we try to "get theology" out of the letter by grabbing a sentence and setting it up as Truth, since it might be in the mouth of one of Paul's opponents.