I think I've done all of those at one point or another. What I've fallen into more recently is something that takes more work but I think is better for the user. Basically, you give a small red popover box with a stubby arrow pointing toward the field in question, telling them it's a required field. Displaying it above the field in a bright colour I think gets the point across well without altering the flow of the rest of the document with inserted content or bolded text.
It gets more complicated if you have multiple errors at which point displaying multiple popovers is very cluttery, so you either decide to only display the first error or only display one error at a time and once one of the conflicts has been resolved display the next error.