The Hunting of the Laugh

Comedy is like sex.  The more comedy you get, the more you want.
Hang on... I'll start this again.
Laughter is like sex.  The more laughs you get, the more you want and the less satisfied you become with perfectly good laughs because they're not as exciting as those first laughs...
Okay... let's give up the sex analogy thing, just not working.
I've just finished performing in a run of She Stoops to Conquer.  It wasn't a long run, five nights, which meant there wasn't much time to hunt for the laughs.  It is an addictive business, laugh hunting.  You're looking for the maximum return of laughs per line, per action - so long as you don't fall into the trap of simply mugging for the sake of it.  That way madness lies.
Some lines are nearly guaranteed for laughs.  They're called jokes.  You set 'em up, I'll knock 'em down.  So long as you don't completely bugger them up, you'll get a laugh.  Rather than worrying about getting the laugh, you are more worried about the size and spend a lot of time worrying about the mechanics of the line.  Will a stress on this syllable get a bigger laugh than on this one?  (Again, that way madness also lies.)
Then there are the lines that are not inherently hilariously funny but in context can be funny.  I spent most of the week trying to squeeze at least a titter out of these.  But, until the last night, two titters evaded me.  And I didn't know why.
This is the exchange in the script (I played Hastings):

HASTINGS. Well, but where have you left the ladies? I die with impatience.
TONY. Left them! Why where should I leave them but where I found them?
HASTINGS. This is a riddle.
TONY. Riddle me this then. What's that goes round the house, and round the house, and never touches the house?
HASTINGS. I'm still astray.
TONY. Why, that's it, mon. I have led them astray...

Not an inherently hilarious section, but with distinct possibilities.  Now, I knew from rehearsal that if I put a slight beat in before the line 'This is a riddle' and the following 'I'm still astray' I should get a slight laugh.  Not a big laugh, but a little laugh.  Tony, it has been established is a character who doesn't quite say what he means and certainly not quickly or clearly - a moment to think about what he said and then saying so seemed moderately amusing.  
In rehearsal the beat worked - I got a titter or two from the cast, so I knew I wasn't barking up the wrong tree.  First night.  Nothing.  Second night.  Nothing.  Third night.  Nothing.  Fourth night.  Nothing.  I said the line in different ways, varied the pausing, tried it as an aside to the audience, adjusted the physical scoring of the beat... NOTHING.
So, in desperation, I did something a bit unprofessional and made a request of my fellow performer.  I asked Tony to change the blocking slightly and look at me on the proceeding line.
And, dear reader, we got the ghost of a titter.  On that first line, just the ghost of a titter.  And on the second, a genuinely tiny but actually measurable one.
We may not have got the complete laugh, but at least I knew, with a few more performances, we would have done.  We understood why it wasn't working (there was not enough connection between what he was saying to me and me to him, basically) and that was everything.
As I say, that way madness lies.