I can't watch the action or the emotions of the characters while reading. I miss half of everything. I don't mind subtitles, but I have to watch the whole thing twice to get the entire story.
When I was younger and first started getting into anime, I remember lowering my eye gaze, reading every word and then raising my eye gaze to catch what little of the scene I could before lowering again to read every word.
But some where along the way, that stopped happening. I don't know why or how, but my brain just got trained into doing both at the same time. I think I just take a snap shot of the words in an instant glance and then the words slowly play out in my head while watching the scene.
But this was after years and years of watching subtitles. It is probably not something that happens quickly for most people.