Lol. This times twelve thousand.
Since most players with interrupts are terrible clickers, their reaction times are slow and they will actually end up interrupting your next cast which you thought would be safe from interrupting. This isn't because they are good, it is actually because they are terrible, but you will still suffer the consequences.
TL;DR: Fake casting is only useful against players that you know are GOOD, and even then, extremely good players have very quick reaction times and aren't fooled by fake casts and only interrupt at about 75% or more of cast time completed.