Speech and Dialogue
Not much of a surprise here since Samson doesn't have a narrator. We pretty much only learn about characters from how they speak. And they tend to speak a lot, so we're in luck.
As Samson receives his various visitors, we can slowly piece together a picture of his character and of his visitors. For example, when Dalila comes to visit Samson, she announces her presence with language that is cautious and uncertain: "With doubtful feet and wavering resolution/ I came, still dreading thy displeasure Samson" (733-4).
By contrast, the giant announces his presence with extreme pride, saying "I come not Samson, to condole thy chance... I am of Gath,/ Man call me Harapha, of stock renown'd" (1076-1079).
And yet despite Dalila and Harapha's distinct proclamations, Samson answers them in a similar tone of anger and disinterest that characterizes most of his interactions: he's made up his mind and chatting about it won't change anything.
Thoughts and Opinions
Considering how little direct action takes place in Samson, thoughts and opinions become practically events in themselves. Take, for example, the differences between Samson and his father, Manoa.
Samson seems firmly committed to the justness of his punishment: "...let me here,/ As I deserve, pay on my punishment" (488-9). Manoa, on the other hand, has a very different understanding of what punishment means: "Be penitent and for thy fault contrarie,/ but act not in they own affliction, son" (503-4).
The point here isn't just that Samson thinks he should stay in prison while Manoa wants him free, but that when Samson commits to a certain point of view, he isn't swayed by emotional appeals. And this is super important, because it shows how much he's changed since the Dalila days.
We get that you're skeptical about how "action" can tell us anything about this text. And we're going to be totally honest: it's not the easiest. But it is important, because Samson asks us to consider how actions reveal information about character. This is why Samson and his various visitors spend so much time examining and analyzing the various choices and decisions Samson made in the past. They, like us, are wondering what kind of guy Samson is, and if we can even tell from the actions he's taken in the past.
Obviously, the biggest action of all in this poem is Samson's destruction of the temple. And while Manoa and the Chorus are all "Go Samson!," we bet you were a little more like "Did he seriously just kill himself and a bunch of other people? Is that really heroic?"
While we don't think there's one easy answer, you're on the right track if you're thinking about the complex relationship between Samson's identity and the things he does.