Then you look around and realize that you can't half-ass anything. Every job you do has to be fully committed to. Whether it's the children's show that runs 30 minutes, the off-the wall comedy where you only appear for Act II, the Shakespeare where your lines consist of "Yes, sir" and a bow, or the commercial where you just have to 'look' worried. Distraction is death when the whole point of acting is to be 'in the moment'.
Everything you commit to, you have to commit to. Otherwise, have a rehearsal where you walk in feeling good about your preparation (half-assed as it was, you did do your work) and walk out feeling like a 1000% twat.
And you know, deep down, that if your life was a TV show, on this day, at least, you'd be Trevor.