Keep your mouth shut and see what's happening around you. Don't finish people's sentences for them. Don't just hear what they say, but also how they behave while they're saying it. That was great training for writing.
I get to tell the most interesting stories I know how to tell with the most interesting sentences I know how to compose - and people who aren't related to me read them. To be paid to write things that matter to me is extraordinary.
If the characters are not alive to me, it doesn't matter how good the sentences are. It just becomes all cake and no frosting.