Matt pretty much covered it. There are a couple of exceptions to be aware of.
- While using "bat hits ball versus ball hits bat" to determine interference will work 99.99% of the time, there is one rare exception.
If, in the umpire's judgment, the batter willfully placed the bat into the path of the ball, for the purpose of intentionally deflecting the ball, then in that one case even if the bat is lying still on the ground and the ball hits it, interference can be ruled.
- On Matt's last point, I don't think that "intentionally" is part of the equation. You have either interfered, or you have not, and can be guilty of interference even without intending to.
Here is the Case Play for rule 7-4-14:
SITUATION: In hitting a slow roller to F5, the (a) whole bat slips out of
B1's hands and interferes with F5 or (b) her bat breaks and hits the ball or F5 as
F5 attempts to field the ball. RULING: In (a), the ball is dead immediately. B1 is
declared out for interference, because B1 is responsible for controlling her bat
and not allowing it to interfere with a defensive player attempting a play. In (b),
there is no penalty and the ball remains live.
In part (a), the bat has clearly interfered with a fielder by accident (no intent) and it is still interference.
Part (b) covers a different point that is kind of on a tangent to this rule. It is telling us that a broken bat can never be ruled interference. The whole bat must interfere with a fielder to be a violation.