Kicking this up to the top level and out of the comments so people can join in.
The topic is Improvising in Shakespeare's work. Or, more generally, let's call it "going off script", since it doesn't have to be extemporaneous for our purposes. We're talking about when actors, in between their Shakespeare lines, add the occasional words of their own devising.
I have two thoughts on the subject. First, on the subject of "Do we think that Shakespeare's actors improvised?" I answer, "Shakespeare's not here anymore to defend himself." So I have to assume that, when it was live, he had least had the option of going up to an actor afterwards and saying "That was good, keep it" or "Well, that ruined the show, thanks a lot. Don't do it again." Who really knows if the plays were the same night after night? Shakespeare could have constantly been revising. So while the Works as we've come to know them are like Scripture to us, we almost certainly hold the source material in a much higher regard than the creator did.
Second, I think there is an important distinction between a director saying "Ok, in my vision of the play, I'm going to have you do the following...." versus an actor just deciding to say something funny. I've actually just remembered a good example - during the Commonwealth production of Shrew in Boston several years back, I can't remember why exactly but there's a chase scene - some servant who has impersonated someone is now being chased by that man's bodyguards - anyway, he jumps off the stage and into the audience, turns back to the stage (where the bodyguards are approaching), puts his arms up and yells "Wait!! Fourth Wall!" They pause, confused, just long enough for him to head for the hills, before they too jump down and pursue.
I don't recall at the time being pissed off that the director had thrown this in. I remember thinking it was very funny. It was a directorial decision, and showed some purpose.
Now instead compare a hypothetical scene from Macbeth, at the dinner party before Banquo's ghost makes his appearance. The seated guests are all no doubt socializing and talking amongst themselves, and then one of them pipes up loud enough for the audience to hear, "Rectum? Damn near killed him!" and everybody has a big guffaw.
I think I'd be upset about that.
Are my feelings on the subject arbitrary? I honestly don't know. Could be. Could entirely be in the hands of the particular director or actor. If I get the feeling that the director and/or actors have love and respect for the material and are merely trying, in their own way, to present it in the best possible way? I like that. If on the other hand it seems to me like they've taken the "We need to make this better" approach, then I have a problem with that. And I do realize that this is entirely opinion - Julie Taymor could have nothing but the utmost respect for Shakespeare's work, and this is simply her way of expressing it. I have no idea.