It is very helpful to have a transcript when creating post-production captions if the transcript can be uploaded and auto-timed (as with YouTube captioning).
If a 3rd-party vendor creates captions, be sure to review those captions before distributing the video. Even diligent captioners can make mistakes or misinterpret specialized terminology or names.
While auto-captions are not sufficient on their own, they can provide a good first pass when creating captions. Once auto-captions are available, they can be edited and fine-tuned to create a fully accessible video.
If an event is scripted, a captionist can pre-program the script to have it shown line by line during the event. If the speaker goes off-script, however, the captions will not match the audio. If there is a chance a speaker will go off-script, it might be better to let the captionist provide live captions instead of working directly from a script.
3rd-party captionists will often request material they can use to prepare for live captioning events. Contextual information, speaker names, and specialized vocabulary are all helpful so a captionist can familiarize themselves with the content of the event and pre-program names and terminology into their captioning dictionaries.