I'm confused about your perspective that Jensen Ackles gives over to the other actors and responds accordingly. To me, he seems like he is acting with himself. Or maybe it is the way he is written. Sam, for example, is working on a case with research, as Dean walks into the room. Sam tells him what's up, expressing interest in the lore and so forth. Sam gets a little excited, and I always find that endearing. Dean makes some sarcastic or unrelated comment. Or he's annoyed about something. I feel as though Dean's character was just a series of one liners, where he is basically talking to himself and drawing attention away from the other characters.
Would you point to a scene that illustrates your perspective? I am willing to concede that over time I found Dean increasingly shallow and self-centered. Sometimes, I wished he would just go away so the other characters could get into the story and relate naturally to each other.
I'll give you two scenes. First, the excellent Carry On scene, it was one of Jensen’s best performances because he's always his strongest when it’s about Dean and Sam’s angst. The shock of the traumatic wound kept most of the pain at bay,but Dean could feel his life draining away so he took several shallow breaths to draw more oxygen in to keep himself going because he wants to spend the last few remaining minutes with Sam and make sure his baby brother will be strong enough to move on. Jensen kept his voice low, even, and soothing because Dean needs to keep calm to keep Sam from losing it completely, and to save his strength to focus completely on Sam. Jensen performed masterfully by eschewing most of his acting tics except for his expressive eyes and nuanced facial expression. Gone is Dean’s bravado, Jensen distilled Dean down to his core and we saw his vulnerability when he begged Sam to tell him it’s going to be okay, his faith in Sam’s strength, and his last breath. He can finally rest and be at peace.
This method worked great for Jensen when his character has Sam to focus on, and Jensen has Jared to react to. It’s why Dean’s dying moments with Sam in the barn works so well in the series’ finale.
More importantly, the buildup to Dean's long goodbye speech was organic. Fans rightly criticized his 15x09 angst speech because there was no build up to it, so it comes off as over acting (x). The writing for Dean leans towards inconsistent so Jensen can sometimes appear unsync or overacting because there wasn’t an organic & consistent lead up. It’s why Jensen’s best acting are his scenes with Jared because the writing for Sam is usually consistent (x) so Dean’s reactions make sense to the audience.
Another of Jensen’s best scene was his monologue over Sam’s corpse in season two. Though Dean have already lost his father, we assume that our parents will go before us, that is the natural order, and we expect the eventuality of losing a parent. But we often don’t prepare to lose a sibling, especially a younger sibling whom we assume will outlive us. We also often treat our younger siblings like practice kids and emotionally we sometimes see them as eternal children. For a parent to lose a child is unspeakable hell on earth. Dean is hit twice with tragedy: losing a brother and losing a pseudo son. Jensen displayed Dean’s despair first through his cold indifference to the fate of the world. He’s done everything right and he still lost, the world took away Sam and now Dean owes them nothing. But did he do everything right? Sam still died. Dean emotional self-flagellation over his own failings instead of blaming the world drives him into a place void of hope. He is raw, lost and alone, without purpose and meaning. Without hope, Dean damned himself into a desperate act that is both selfish and selfless and Jensen totally sold it.
