This is the question I don't know myself how to answer. I wouldn't put john@company.com, at least in complex forms. On the one hand you provide sample data, bu

This is the question I don't know myself how to answer. I wouldn't put john@company.com, at least in complex forms. On the one hand you provide sample data, but on the other hand... for example you'll need to provide a fake phone number, some fake data that is not personal. I tend to leave inputs empty (while having labels above them!) and only if they have some kind of masks I show the placeholder. Any hints about how to fill the form I place BELOW the input, so that when user starts typing, they don't disappear. So looks like I just keep them empty most of the time. Or, if it's a super simple form (e.g. only email on landing page), I put "Enter your email here". Since it's kind of call to action, like "do this!".