The book Robert Frost on Writing contains a conversation on the craft of poetry between Frost, Robert Penn Warren, Cleanth Brooks, and Kenney Withers. Here is an excerpt (pp. 155-156):
WARREN: Well, I'm sure you're right about the dramatic quality being the basic quality of good poetry. That would bring up the relation of meter and rhythm to the dramatic moment -- moment by moment -- in a poem, wouldn't it?
FROST: That's right.
WARREN: I'd like to hear you say it in your way, how meter enters into this picture -- the dramatic quality.
FROST: The meter seems to be the basis of -- the waves and the beat of the heart seems to be basic in all making of poetry in all languages -- some sort of meter.
WARREN: The strain of the rhythm against the meter. Is that itself just a dramatic fact that permeates a poem?
FROST: From those two things rises what we call this tune that's different from the tune of the other kind of music. It's a music of itself. And when people say that this will easily turn into -- be set to music, I think it's bad writing. It ought to fight being set to music if it's got expression in it.
WARREN: Yes, there's something resistant and unique in it; you can't turn it into something else. This is to overstate the matter, but I do want to get it clear, if I can for myself: would you say that even though the meter is based on the human pulse or some kind of basic rhythm in our natures, still for the poet it's something to be played against -- it's not something to be fought with, to be tussled with? It's not directly expressive -- ta-DA, ta-DA, ta-DA, ta-DA, ta-DA.
FROST: No, it's doggerel when you do that. You see, and how you save it from doggerel is having enough dramatic meaning in it for the other thing to break the doggerel. And it mustn't break with it. I said years ago that it reminds me of a donkey and a donkey cart; for some of the time the cart is on the tugs and some of the time on the hold-backs. You see it's that way all the time. The one's doing that and the other -- the one's holding the thing back and the other's pushing it forward -- and so on, back and forward. I puzzled over it for many years and tried to make people see what I meant. They use the word "rhythm" about a lot of free verse; and gee, what's the good of the rhythm unless it is on something that trips it -- that it ruffles? You know, it's got to ruffle the meter.
Now, I find this interesting because I think Warren and Frost are getting at something -- the interplay of meter and rhythm -- that holds true for music as well. The best expression I've found of it is in Victor Zuckerkandl's book Sound and Symbol (p. 172):
Such is the case in all metrical music. To put it metaphorically: the ground upon which the tones fall is itself in wave motion. The wave is the meter; rhythm arises from the different arrangements of the tones on the wave. The greatest possible latitude is accorded to the nature and manner of these arrangements. The tones may be distributed over the measure regularly or irregularly; may fill the measure in rapid succession or leave it empty for long stretches; at one place crowd close together, at another spread thin; may follow the pattern of the measure with their accents or run contrary to it. This freedom of distribution and arrangement makes it possible for the tones to give the constant basic form of the wave a changing, perpetually different profile. In accordance with the will of the tones, the wave will display contours now soft and rounded, now sharp and jagged; will beat softly and calmly or with ever-increasing impact; will heave, topple, break against resistances. This playing with the wave by the tones, this shaping of the substance of the wave; the conjunction and opposition of two components, their mutual tension and continuous adjustment to each other -- this, in music, we experience as rhythm.
In both poetry and music, the meter provides an underlying wave of forward motion -- iambic feet and duple time being the most familiar (interestingly, most poems in the English language put the accent on the second beat, whereas most metrical music puts the accent on the first beat). But if one relies too heavily on mere meter, the result is poetic doggerel (ta-DA ta-DA) or unsophisticated music (the OOM-pah OOM-pah of bad polkas). The dramatic possibilities of rhythm are realized when the words or tones ruffle the meter, resist it, play with it, push it forward and then hold it back, break with it without breaking it.