I know that when a host and symbiont are joined, they're supposed to be a "synthesis" of the two life forms, where the host would absorb the memories and other aspects of previous hosts, but how "fused" were they? Was there any autonomous or independent thought from one or the other at all? Just as an example, when Jadzia Dax fulfilled Curzon Dax's blood oath to avenge Kang's son. What if Jadzia figured it's not my problem. That was Curzon Dax's promise, not Jadzia Dax's promise. Has there ever been any indication the symbiont or host could, for lack of a better word, disagree over something like that? Not in word, or course, but maybe just emotionally. Like a part of you doesn't want to do it, while another part feels an obligation to do it.
I'll just point out that (Facets establishes that) Dax has memories that it chooses not to share with Jadzia and leave it at that.
Without spoiling, DS9 kinda waffles on how exactly Trills work and all, especially if you compare the first couple seasons with the last couple of seasons.
DS9 went for a more "balanced" approach compared to the Trill's introduction in TNG, since they were only an alien of the week and the symbiont's personality was clearly the dominant one. When Riker was joined with Odan's symbiont, he took on elements of Odan's core personality and the same was true of the replacement host. It was a bit ambiguous initially about how a newer host would deal with "remnants" (for lack of a better term) of things that occurred in the life of a prior host. The episode "Dax" deals with whether Jadzia could be accountable for actions Curzon took, and I seem to recall that under Trill law different hosts were effectively separate individuals (though my memory may be faulty ) and were not liable for such actions, even if they carried the memories of those events via the symbiont. Jadzia did mention that at times she felt memories and aspects of former hosts that didn't feel like herself, like the Jadzia self, and that it could be a bit disconcerting.
In my opinion early Jadzia Dax was a lot more Dax than Jadzia, while later on she was a lot more Jadzia than Dax.
Based on her description of what Jadzia was like before joining, I'd argue it was the other way around. That the Dax element brings about a wild side, a zest for firmly embracing all aspects of life with both hands.
Originally, the Trill symbiont was supposed to be the dominant personality. Later on, it became the host. Which, given the competition for the privilege of being joined, makes far more sense. What's the point of immortality if you're not really you anymore?
I sometimes wonder if the Trill would have worked better if all Trill (or at least the majority) were naturally joined, as might have been the original intent when "The Host" was written.
The thing is, the symbiont could always fake the "free will of the host" by forcing the host to have suitable free-willish expressions when needed. The relationship could be pure slavery for the bodies that the slug possesses. We, and you, have seen Jadzia freed of Dax for a brief moment in "Invasive Procedures". She didn't start screaming "I'm free! I'm finally free! Gimme a protoplaser and I'll make that slug suffer!" while recuperating from the de-slugging operation. Then again, she was no doubt heavily sedated, and about to die of an acute lack of symbiont; not a particularly reliable witness. More reliable evidence that the symbionts aren't maliciously possessive comes from those who temped as hosts: Riker, the man who stole Dax in the above incident, those who assisted in assorted rituals... But a symbiont might choose to behave when aware that the host will get an opportunity to testify. Indeed, I'd presume an element of choosing there, with personality of the symbiont playing a major role in the choices it makes. Odan from TNG would simply be an asshole who doesn't respect its hosts enough to use their names in connection with its. Dax would be an old codger who delights in borderline-suicidal behavior because it has already seen it all (and dying is always so exhilarating, or indeed trilling!). And the hosts go along with it, more or less willingly, but the symbiont always has the upper hand, being in some sort of a chemical connection with the host body and probably capable of tempering its responses, toying with hormones and glands, and in general making it worth the while for the host to behave. Timo Saloniemi