Some scenes can come back and haunt an actor years later, but that’s not quite the case for Keira Knightley. While she doesn’t regret it, it wasn’t something that she’d do again if she had the chance. That’s generally the way these things tend to work, right enough. Something seems like a good idea at the time, and even after the fact, it isn’t worth being ashamed of, but the passage of time, the benefit of hindsight, and the rest of the wisdom …