One of the things I write about a lot is the importance of having your character make big, bold choices. Those choices teach us who they are and give us a reason to root for them. I think it may be the most important thing in a pilot; you'd think it would be have a great character, and that's clearly important, too. But if they don't make any big choices, I'm not sure it matters.
At the end of STRANGER THINGS 406, we see two versions of that principle at work. One is really effective, and the other sort of challenges my whole concept.
Let's start with that. The older kids go out in the boat to investigate a rupture between the universes. And when they get to the spot where it seems to be coming from, Steve agrees to swim down and check it out. It's definitely a bold move. But he does so without any reflection on the danger involved. And what's more when he seems to wake something up on the other side, he seems genuinely surprised--and then is surprised again when that thing drags him down.
None of that makes any sense. No one in that situation reacts that way. And so in that moment, rather than us feeling more connected to Steve, we feel removed from him. His bold choice is presented in a way that actually undermines his credibility; it's very much what the writers needed to happen.
But then on the other side of things, once Steve is dragged down, the other characters all jump in after him. Which is equally foolhardy, obviously, but now it's done with the acknowledgement of them knowing that. This isn't about being clueless, it's about saving their friend. It's the very definition of a great, character-defining choice.