Speech Stands Alone?

It was a suggestion, and I think that you should take it with a grain of salt, but it is a good idea to avoid adding action by someone else to a piece of dialogue.

I find it good advice. I often tell authors I am reviewing that 'good dialogue doesn't need stage directions'. However, it is only a suggestion, and not a rule.

~~A more careful reading of your comment brings up a question. "I know about vampires and everything," he flinched ~~with the speaker not mentioned? Is this the sort of dialogue add-on you mean? If so, I would say yes, keep one character's actions out of another character's speech. As far as I'm concerned, the rule about one character's thoughts, actions and speech per paragraph covers this question.

If you will insist on having the listener's reactions right next to what the speaker says, try this construction: "I know about vampires and everything," I said, and watched him flinch. ~~at the very least, it may confuse your critics.

It's not set in stone, but it is very good advice.

I've had to point this out more than once when beta-ing. It can cause dire "who-said-what" confusion.