Double Star by Robert A. Heinlein

is simply a matter of inner attitude; it has nothing to do with the steady march of catabolism.
                I could have played him on boards, or read a speech in his place, within twenty minutes. But this part, as I understood it, would be more than such an interpretation; Dak had hinted that I would have to convince people who knew hlin well, perhaps in intimate circumstances. This is surpassingly more difficult. Does he take sugar in his coffee? If so, how much? Which hand does he use to strike a cigarette and with what gesture? I got the answer to that one and planted it deep in my mind even as I phrased the question; the simulacrum in front of me struck a cigarette in a fashion that convinced me that he had used matches and the oldfashioned sort of gasper for years before he had gone along with the march of so-called progress.
                Worst of all, a man is not a single complexity; he is a different complexity to every person who knows him-which means that, to be successful, an impersonation must change for each "audience"
    -for each acquaintance of the man being impersonated. This is not merely difficult; it is statistically impossible. Such little things could trip one up. What shared experiences does your principal have with acquaintance John Jones? With a hundred, or a thousand, John Joneses? How could an impersonator possibly know?
                Acting per Se, like all art, is a process of abstracting, of retaining only significant detail. But in impersonation any detail can be significant. In time, something as silly as not crunching celery could let the cat out of the bag.
                Then I recalled with glum conviction that my performance probably need be convincing only long enough for a marksman to draw a bead on me.
                But I was still studying the man I was to replace (what else could I do?) when the door opened and I heard Dak in his proper person call out, "Anybody home?" The lights came on, the threedimensional vision faded, and I felt as if I had been wrenched from a dream. I turned my head; the young woman called Penny was struggling to lift her head from the other hydraulic bed and Dak was standing braced in the doorway.
                I looked at him and said wonderingly, "How do you manage to stand up?" Part of my mind, the professional part that works independentiy, was noting how he stood and filing it in a new drawer marked: "How a Man Stands under Two Gravities."
                He grinned at me. "Nothing to it. I wear arch supports."
                "Hmmmph!"
                "You can stand up, if you want to. Ordinarily we discourage passengers from getting out of the boost tanks when we are torching at anything over one and a half gees-too much chance that some idiot wifi fall over his own feet and break a leg. But I once saw a really tough weight-lifter type climb out of the press and walk at five gravities-but he was never good for much afterwards. But two gees is okay-about like carrying another man piggyback." He glanced at the young lady. "Giving him the straight word, Penny?"
                "He hasn't asked anything yet."
                "So? Lorenzo, I thought you were the lad who wanted all the answers."
                I shrugged. "I cannot now see that it matters, since it is evident that I will not live long enough to appreciate them."
                "Eh? What soured your milk, old son?"
                "Captain Broadbent," I said bitterly, "I am inhibited in expressing myself by the presence of a lady; therefore I cannot adequately discuss your ancestry, personal habits, morals, and destination. Let it stand that I knew what you had tricked me into as soon as I became aware of the identity of the man I am to impersonate. I will content myself with one question only: who is about to attempt to assassinate Bonforte? Even a clay pigeon should