"[B]eing at one with oneself is an ideal with its roots in animal life", writes Terry Pinkard (Hegel's Naturalism, p 58), expressing an opinion which I too have long held . (In fact, concern about some such idea has shown up in versions of research statements of mine, though such a concern has yet to manifest itself in much actual research, excepting an APA presentation on Schlegel and Nietzsche.) Why in animal agency? Because (again quoting Pinkard) "the animal acts in terms of the principles of its own nature", because (in a not uncommon trope that I always associate with Uexküll, since it was in him that I first encountered it) in the world of the animal there are only that-kind-of-animal-things, things whose significances are plain, and because for the animal there is no distinction between things as they are for it and things as they are in themselves. (Even the lark, says Heidegger, does not see the open.) The hungry animal seeks food, the endangered animal shelter, etc.; in their actions they do, as we would put it if talking of ourselves, what comes naturally. The only reason not to put it that way when talking of them is the suggestion that an animal might do something that does not come naturally, might do something not of its nature. (Uexküll, at least, was willing to recognize some plasticity in the natures of e.g. dogs; it's in their natures to acquire new objects, but even for him dogs are not able to see their objects as objects whose objecthood might thereby come into question.)
This is not to say, of course, that an animal always attains its end, or that every animal exhibits surefooted mastery of its environment, an impossibility given both predators and prey. Only that, on the one hand, its end is neither a question for an animal nor set before it as end, and on the other, its not having achieved its end is not something that could elicit a response such as: "Curses, foiled again!". One is at one with oneself at least partially in many accounts of absorbed activity, the fluent action (often confused with basic action, both by proponents and opponents of basic action) said to be characteristic of craftsmen: one is so to speak led on smoothly from one end to the other, successive actions being elicited by the objects worked upon, tools worked with, etc., the agency so to speak dispersed in the environment in a manner analogous to that of an animal—so long as nothing breaks down. But, of course, breakdowns are possible, and if one occurs, one will be called back to oneself, what one was doing now present to oneself as the unattained end. One could conceive of another model here, supra- rather than infrahuman, in which no breakdown is possible, not because the material is flawless, but because the actor is prepared in advance for all contingencies. No breakdown is possible for an animal because, no matter how unfriendly or alien the environment, its ends cannot be made problematic for it; no breakdown is possible for a god because no environment can be unfriendly or alien enough to recall it from action to contemplation. (Having the end is the same as accomplishing it. I suppose this elides the question of determining the end in the first place.)
Not only animals and gods have been thought to be at one with themselves in something like this fashion. The ancient Greek, for instance, "had not lost nature in his humanity" and was "at one with himself and happy in the sense of his humanity", says Schiller. Robert De Niro  "flows into his movements, even the most basic ones. Opening fridge doors, lighting cigarettes. He doesn't have to think about them, or understand them first. He doesn't have to think about them because he and they are one. Perfect. Real.", says the narrator of Remainder (p 24), having previously observed that De Niro "merge[s] with [the action] until he was it and it was him and there was nothing in between" (p 23). And just a page prior to that the narrator remarks that most of us—he formerly included—are like that with our basic physical abilities:
Walking, for example: now that's very complicated. There are seventy-five manoeuvres involved in taking a single step forward, and each manoeuvre has its own command. I had to learn them all, all seventy-five. And if you think That's not so bad: we all have to learn to walk once; you just had to learn it twice, you're wrong. Completely wrong. That's just it, see: in the normal run of things you never learn to walk like you learn swimming, French, or tennis. You just do it without thinking how you do it: you stumble into it, literally.
The narrator gradually regains his ability to walk without thinking of walking, but after beholding De Niro's perfection on the screen recognizes that in his larger deeds he's still as he was before his accident (one of the things he recognizes is that he was thus before his accident and not merely after):
Recovering from the accident, learning to move and walk, understanding before I could act—all this just made me become even more what I'd always been anyway, added another layer of distance between me and things I did. … I wasn't unusual: I was more usual than most.
The ambition and attempt somehow to overcome the state that he and more or less everyone else is in occupies, in one way or another, the rest of the book. At first he just looks for people who aren't "artificial", as he puts it; then, seeming to remember a non-artificial (but also nontrivial: not just walking) act of his, he seeks to reproduce it—on its face an odd approach since one dimension of what he finds objectionable is precisely the doubling of action in thought of action (lighting the cigarette perfectly, and thinking: "I'm lighting the cigarette perfectly", or even "I'm lighting the cigarette perfectly—like De Niro!"). But in fact something like this is the only reasonable way forward. After all: that's how we learn to walk, isn't it?—constant trials, until it's second nature.
Let us not concern ourselves with his many reenactments of scenes remembered or witnessed and concern ourselves henceforth with his original works—the bank heist and the back-and-forth plane flight. First, however, let us return to the imagined god-like Beisichseiende.
What makes the being of that being unattainable for beings such as we are? It's not for nothing that leading examples of fluent action occur in constrained realms—generally either the movements we use just for getting around, or some set of movements around which some kind of specialized practice has grown up: crafts, or sports, or the arts. (Not just movements, of course; these things involve, for instance, recognitional capacities as well. The ability to hear spoken English with comprehension, or, I suppose, tell at a glance that the jam has just set, for instance. But these still occur within constraints.) Importantly, first, when you've got some kind of practice, some things fall outside the practice and can be discounted; second, because the practice is set off within all of life, episodes in the practice can be returned to and events or actions within it reprised and, well, practiced. You can practice your fingering, or your performance of a given piece, and get better at it. And if, in the middle of a given performance, the ceiling collapses and you're knocked over by falling masonry, that does not reflect poorly on your abilities as (e.g.) a violinist: dodging falling objects is simply not within the violinist's remit. If you face a difficult decision in a chess game, you can, after losing, revisit the game (if you weren't recording it, you can revisit that decision) and see what responses were available to you at that point (or earlier). When you play your next game, you're starting over from the beginning: the games are disjoint events. The second doesn't happen later than the first in some scale of time native to chess. Consequently the same thing can happen again, and you can be better prepared.
You can't be practiced at life that way, something that people who recommend artistic self-fashioning would be advised to bear in mind: there are no trial runs, no repetitions of little bits that you can get better at, no seeing what went wrong after the fact and working out how to behave should the same thing occur again—it can't. The idiom which the genius of living would have to master includes every eventuality, falling ceilings included, and would have to get it right the first time, every time. Even if we discount a period of maturation, we could not possibly become able to encounter anything without breakdown, because there are just too many surprising things. (A theme that Millgram sounds vigorously.) Consequently we cannot expect that we, given that we are capable of the reflection and explicit bringing into view of ends as ends that characterizes breakdowns (i.e. given that we are not animals), will never actually be brought out of ourselves into such reflection (i.e. we are not divinely self-sufficient). This is also why I suspect the ideal of being at one with oneself of being a flight from the difficulties of the distinctively human . (I once asked Rachel Barney about something not entirely unrelated to the preceding after a talk about the Stoics. She noted, in her reply, that the phronimos is rarer than the phoenix, and that there's only one of them at a time.)
De Niro can merge with all his actions (onscreen) because, duh, it's not really happening and do-overs are allowed. Outside of more or less artificial circumstances we cannot, in general, achieve a like merger with our action. The environment is, at least potentially, too recalcitrant. This is neatly dramatized by the bank robbery. The thing is rehearsed until all the participants have their parts down pat; these rehearsals happen, like practice chess games or any other rehearsal, in their own time, embedded within the lives of the pre-enactors. When they go out to actually rob the actual bank, they will, it is hoped, be perfectly at one with their actions, which in the interim will have become second nature to them. And there are some things that could conceivably arise (John McClane happens to be in the bank; a freak natural disaster sweeps through) that would not impugn their status as excellent robbers. In that respect the narrator's ambitions are realistic: the people he's assembling aren't to gain mastery at everything, but only in a limited arena. This in fact comes up explicitly in the early planning stages:
"I say 'ideally'," Samuels continued, "because this pattern is to both sides' great advantage. The robbers get their money and the bank staff don't get killed. What messes it all up is when a factor no one has anticipated and built into the pattern breaks in. … A have-a-go hero jumping one of the robbers, a hysterical woman who won't obey commands, someone who tries to run out fo the door …" (p 250; the second ellipsis is in the text.)
An extremely limited arena, in fact; the potential recalcitrance of the environment to our plans is demonstrated by something quite minor. During an early rehearsal (perhaps the first), one of the pre-enactors trips over a kink in the carpet. This is incorporated into the act and the pre-enactor becomes practiced at stumbling and catching himself. When rehearsals end, though, and the act is taken on tour to the actual bank, the carpet is flat, and the whole thing goes agley. The pre-enactors are brought out of their practiced movements and must come to understand the situation they now find themselves in before they can act, choosing to abandoning the ends they were previously absorbed in (one of them calls out for the "re-enactment" to stop, not yet having figured out that it's not a re-enactment and the others in the bank are unwilling co-participants).  It is appropriate that the narrator ends up in the highly constrained environment of an airplane, flying back and forth without setting down: here, at least, is an environment not specially set apart from the rest of the world which he can fully take in. (Until, after the book ends, the issue of fuel becomes pressing.)
 Obviously not only I; many have held it for much longer: "We love in [animals etc.] the tacitly created life, the serene spontaneity of their activity, existence in accordance with their own laws, the inner necessity, the eternal unity with themselves", quo' Schiller.
 I thought, prior to checking, that the narrator has this thought about John Wayne.
 Perhaps I should add "when this is individualistic", at least until I finish Pinkard's book—though societies also clash and I can't say I've ever been incredibly satisfied with e.g. McDowell's explanation of Aristotle's seeming lack of curiosity about, say, the customs of the Massagetae.
 I don't believe I noted this before but the narrator, throughout all of this, is in just the state of mind that he disliked at the beginning of the book, standing apart from it all and assessing it. I suppose that he more wants to observe fluid action than to partake of it himself—or he's in the double bind of wanting to partake of it and to appreciate himself partaking of it. He also, in his assessment of his fallen employees ("I looked at Two and Five lying on the floor. They seemed now less like acrobats than sculptures. The bag that had slipped from Five's hand and the gun that now lay beside Two's looked to me like wedges of surpluis matter stripped away to reveal them."), reminds me more than a little of Rönne: "Trotzdem verrichtete er weiter, was an Fragen und Befehlen zu verrichten war; klopfte mit einem Finger der rechten Hand auf einen der linken, dann stand eine Lunge darunter; trat an Betten: guten Morgen, was macht Ihr Leib", in "Gehirne", "Auf allen Tischen standen Geräte, welche für den Hunger, welche für den Durst", in "Die Eroberung".