A target_print entity allows you to have text ingame, link it up to a trigger once. I tried to do text in cut scenes but had no luck at all, didnt seem possibly, with any method anyone could think of. If anyone finds a way of doing it I'd love to know, text in cutscenes would be great.
As for subtitles, it seems they are linked to the actual sound files or something somehow, but I never worked this out either! There was a thread on this before, sorry but I can't find it currently