R3 "interferes" with F5 on line drive

[David Emerling]

Originally Posted by Rich Ives

Nope. You dive back just like a pick at 1B. Stay foul - reach to the edge of the bag. Keeps you farther from the tag attempt that way and prevents getting hit by a fair ball too. And you said that R3 dove.

Actually, runners (on 3rd) are taught to take their leadoff in foul territory (so as not to get hit by a fair batted ball, like you said) but to return in FAIR territory (after the pitch) so as to block any attempt on the part of the catcher to pick him off - forcing the catcher to throw around him or, better yet, the thrown ball hits the runner and is deflected away allowing him to score. "Lead off in foul, return in fair" is the proper way.
This is proper technique: https://www.youtube.com/watch?v=YaE-725FNCU

In any case, none of this is the point of the question. An answer to a rules question is never, "That would never happen" or "The player shouldn't do it that way." There should always be an answer; because weird things do happen and players sometimes don't do things the way they should.