Greco-Roman letter writing and the constraints or creative freedom of an amanuensis?

On questions of authorship, some scholars will appeal to an "amanuensis" or letter-writing assistant to explain differences in style, grammar, or language between documents supposedly written by one author. My question is: what exactly was the normal role of an amanuensis in this setting, and would they not have just written down what was dictated to them verbatim, or would they have had the creative freedom to put the ideas of the author into their own words, thus explaining some grammar/vocab differences?

Thanks for the input.

(Posted this in r/AskHistorians and got nothing so far)