The words "deterministic" and "nondeterministic" or "acausal" get thrown around a lot and I would just like to take a moment to explore the ramifications a little bit. Generally speaking, when people use the first word, they mean that the laws of causation -- of cause and effect -- are firmly in place and that in theory causes and effects can be wholly understood to describe an entire situation (such as the entire universe). I pause here, because in reality, aren't cause and effect fictions of the human mind designed to deal, in the abstract, with phenomena which are thoroughly non-constrained by these definitions? At bottom, there are no causes or effects, just a swirl of energy that is undifferentiated in truth but in practice only so in the mind of an observer. Since Quantum Theory entered the picture, the philosophy of cause and effect has differed wildly from its previous incarnation while nature has remained ever the same. Now we have "nondeterministic" or simply "acausal" principles governing nature, even though the entire scope of human use of the convention of cause and effect has gone unchanged. I think for the most part, cause and effect lead to a lot of confusion of language. We are confusing reality with words and it is doing us in. The only sense in which I see "determinism" as a viable concept is the suggestion that the future can be predicted with enough information about the present. Frankly, I don't think such a thing can be done except from a higher dimension of reality, but in principle, in the context of positing Block Time, it makes some sense. However, I would like to add that in block time, and in a "deterministic" universe, free will can still exist and is quite likely.
"Cause" and "effect" are convenient fictions, they have no ultimate meaning -- only the proximate conventionality of association -- and yet they are the basis of millions of personal philosophies of nature. To posit them affords no true understanding whatsoever; they are conceptual tags based on the need for a human mind to find some type of relative invariance by which to gain hold of some fact mentally. These ideas are exclusively that: ideas. They do not connect to points which are meaningful for the cosmos -- merely for man. Nature is one undivided, flowing movement; any grasping at static points from which to conceptualize is nothing aside from a human abstraction.
To actually comprehend the full set of "causes" for even the simplest physical systems is virtually impossible for humans. When one proffers a "causal" theory, such as Many-Worlds or Hidden Variables, I prefer the term "reasonable." The causes of a thing are literally infinitely complex. If the reality of our equations describes a deterministic theory (which is really no different than saying causal), it might be a little more realistic to call it reasonable. We will not soon understand what is really meant by the term "cause" and the literal infinity of which it is constituted. And when we discuss random or "acausal" theories, I think we are safe, whatever their merit, in describing them as unreasonable. Cause and effect, which get thrown around too loosely, are actually beyond human understanding.
Everyone is obsessed with the philosophical diversion that is causality. One can posit a cause or an effect, but such conceptualization affords no real understanding. They're good for emergent phenomena at an everyday or metaphorical level. That's all.
Human concepts of cause and effect will prove to be woefully inadequate in describing levels of Nature beyond the mundane.
Everyone is always arguing about whether the fundamental nature of the cosmos is one of determinism or one of chance. I suggest that both causality and chance are quite necessary for physical and mathematical law, and that perhaps one day we will attain a knowledge which shows us that the two approaches are both valid perspectives on a unified, deeper truth which transcends both.
Causality is the conceptualized behavior we abstract from our observations of Nature in our corner of the multiverse. At a fundamental level, what we think is happening -- what we take for granted -- is not the reality. Our deterministic macroscopic world is really an epiphenomenon of the micro-realm, and gives rise to a bevy of illusions.
Everyone is always arguing about whether our behavior is inherently deterministic or whether it is willfully caused. In truth, it comprises elements of both, all the time. That's why we're so confused -- different perspectives (on the single reality) lead to different conclusions and beliefs. I would point out that, regardless of whether we are fated or free, we do seem to be on automatic pilot most of the time.
Diversity of character is much less rich and mysterious than most people think. It is really only based upon the differential activity of the four "terrestrial" circuits of the nervous system across many individuals. It is, alas, seemingly much more deterministic and much less subtle than is generally presumed, fundamentally.
Everything under the sun is interaction. Materialism, and causality for that matter, is not something real "out there" -- it is only a concept which is useful. And one which it may be time to discard as a culture.
The scientific mindset can undoubtedly be very useful and wonderful, but the fact is that for the most part it is concerned chiefly with materiality. It is classical. Deterministic. The "billiard-ball universe." This centeredness on the material dimension can grow tiresome. Happily, physics has, over the course of the last century or so, discovered (the hard way) that there is much more to nature than just the aforementioned dimension. Popular culture, and even the majority of the scientific establishment itself, have not caught up.
There is in existence only continuity of interaction in time. Cause and effect are abstracted labels which we use to point at various aspects of reality and make conceptual pictures/freeze-frames of that continuous flow of interaction. In reality cause and effect are not fundamental, and do not correlate with objective nature. However, cause and effect do correlate with subjective nature. As emergent phenomena in the human world, cause and effect are very useful and are in fact ubiquitous. I caused the milk to spill. I caused my car to start by turning the ignition. We readily observe how useful this subjective, emergent concept describes our daily lives. While there may be no correlation in nature between the concept and the actual stream of interaction at a basic level, to a human mind emergent phenomena are as real as anything else. So, while it is good to realize that cause and effect are merely artificial labels, it is also necessary to realize that humans are causative agents.
Imagine time as a line, and the next higher dimension as a kind of cylinder surrounding the line radially. Potentially, there could be connections between any point (isolated spacetime state) in the cylinder and any other point in time, at any time. (The idea of a cylinder is used because it surrounds the line symmetrically; it is only an abstraction to illustrate the hyperspace in which acausality can be generated in higher dimensions for our dimension of time). That is, past, future, and basic causality could really cease to have precise meaning in the universe.
I don't believe in a deterministic, "billiard-ball" view of the universe. To me, the universe is more "mind"-like than anything, and can, after a fashion, "think" up novel realities.
The battle between free will and determinism is so hard to resolve because in reality both are true.