Part of the allure of modal actualism is that it makes it easier to state what we mean by truth values in possible worlds. Truth values in an actualist world is evaluated by whether it corresponds with any or some possible world, be they considered as "states of affairs," "sets of properties," "propositions" etc. Since these are actual things that any statement can be compared, one is able to evaluate the truth of falsity of modal statements. Whereas in a possibilist system, how exactly are the truth values of modal statements to be evaluated? Possibilia are non-existent things, so how does one evaluate any modal statement under possiblism?
To answer this question, we must ask ourselves how possible worlds come about. Possible worlds are thought of by someone. Someone wonders what might happen if aliens invaded the earth, and we end up with tons of alien invasions storylines. Some others wondered what a mythology for England would look like, and we arrived at Tolkien's Middle Earth. George Lucas conceived of a possible world with New Age concepts in a sci-fi fantasy setting, and we have Star Wars. The list goes on and on. All of these are possible worlds, conceived by one or more humans using the power of their imaginations. Despite being fiction, they fit any qualification for inclusion as possible worlds, as a giant catalogue of "What If" scenarios play out in the minds of imaginative minds.
What allows some possible worlds to actually seem like a possible universe, while others are absolute trash? Plausible possible worlds are internally coherent. If the world holds that the laws of physics are similar to our own, then the laws of physics will operate just like ours. If the world is a sci-fi setting that holds that there is a miracle ubotainium metal that provides energy and gravitational manipulation, that unobtainium will not suddenly become a human being without powers. In a fantasy setting with magic that is governed by quantity of mana, it is always the case that someone with more mana has access to more magical power. The point here is simple: Any real possible world must be self-consistent. In other words, there is a coherence to any possible world which cannot be violated without undermining that world.
After Disney Star Wars took the liberty of "innovating" Star Wars, Episode VIII The Last Jedi was an absolute mess (I did not even bother paying to watch it; just watch the train wreck later on either Netflix or Disney Plus). One scene in that stinking mess illustrates why coherence in world-building is so important. That scene is the so-called "Lightspeed ramming" by woke feminist 'Admiral' Holdo. In that scene, Holdo sacrificed herself to take out the pursuing Imperial Dreadnought. Her method to do so is to accelerate to "lightspeed," which normally means entering hyperspace. However, by doing so so close to the dreadnought, hyperspace was not entered before the startship hit the dreadnought, destroying them both together with many other surrounding imperial ships.
That scene was one of the things Star Wars fans were furious over, and with good reason. If "lightspeed ramming" were possible, then everyone should be using it and there would be no reason to build any capital ship. One can just program a droid to make the calculations to execute such suicide missions, and the enemy capital ships would be gone. In fact, the supposed existential threat posed by the two Death Stars make no sense. Just program twenty droid ships, make them do lightspeed ramming into the Death Star, and goodbye Death Star! In other words, "lightspeed ramming" violates the internal logic of the Star Wards mythology. JJ. Abrams' forced retcon that the "Holdo manoeuvre" is a "one in a million" thing is a total joke, because one can get around the odds by using droids who are suposed to be able to do the precise calculations faster than any human (including Holdo) ever could.
All of these show us how we can evaluate truth values in possible worlds, if we reject modal actualism. One evaluates truth values by its consistency with the mythos of the imagined world. From a consistency standpoint, "lightspeed ramming" is necessarily false in the Star Wars universe. That it is treated as "true" by Disney Star Wars makes the Disney Star Wars version of Star Wars an irrational world, where nothing in the world needs to cohere with each other. Is that a "possible world"? No, it is an irrational world where "truth values" corresponds merely to the ipse dixit of whoever happens to be the storyteller at the moment. Nothing needs to make sense anymore in such an irrational world, where nothing can work until the narrator decides how they ought to work, for who knows whether a giant Mickey Mouse will suddenly appear out of nowhere, pick up Darth Sidious and swallow him up before instituting a 1000 year of intergalatic peace under Wanda Maximoff?
Truth values are evaluated in possible worlds based upon the internal logic of each possible world in addition to the normal nonmodal predicates, and none of the entities need to exist ontologially for modal arguments to be evaluated. One does not need to think any of them are actually existing, and thus truth claims in possible worlds can be evaluated under possibilism, or in this case, conceptual possibilism.