Can you imagine his surprise when Grant showed up dirty and in a privates uniform. Nowhere near as ceremonial as he must have expected. However Grant’s informality and warmth must have immediately calmed his anxiety.
I think Grant was 'underprepared', in the sense he had literally been 'under the weather' suffering from a migraine, and he also potentially had not foreseen Lee's decision in the moment. Grant, of course, was never one for formal military dress at the best of times and apparently the wagon carrying his dress uniform was nowhere to be found. I do think the element of unexpected informality may have calmed some of the anxiety Lee felt.
In Grant's own words:
"When I had left the camp that morning I had not expected to see so soon the result that was then taking place, and consequently was in rough garb. I was without sword, as I usually was when on horseback on the field, and wore a soldier's blouse for a coat, with the shoulder straps of my rank to indicate to the army who I was. When I went into the house I found General Lee. We greeted each other, and after shaking hands took our seats. I had my staff with me, a good portion of whom were in the room during the whole of the interview.
What General Lee's feelings were I do not know. As he was a man of much dignity, with an impassible face, it was impossible to say whether he felt inwardly glad that the end had finally come, or felt sad over the result, and was too manly to show it. Whatever his feelings, they were entirely concealed from my observation; but my own feelings, which had been quite jubilant on the receipt of his letter, were sad and depressed. I felt like anything rather than rejoicing at the downfall of a foe who had fought so long and valiantly, and had suffered so much for a cause, though that cause was, I believe, one of the worst for which a people ever fought, and one for which there was the least excuse. I do not question, however, the sincerity of the great mass of those who were opposed to us.
General Lee was dressed in a full uniform which was entirely new, and was wearing a sword of considerable value, very likely the sword which had been presented by the State of Virginia; at all events it was an entirely different sword from the one that would ordinarily be worn in the field. In my rough traveling suit, the uniform of a private with the straps of a lieutenant-general, I must have contrasted very strangely with a man so handsomely dresssed, six feet high and of faultless form. But this was not a matter that I thought of until afterwards."
The Complete Personal Memoirs of Ulysses S. Grant - pg 353
And further to Grant's situation:
He had also been feeling unwell the day before and it must have required some effort on his part to meet the occasion.
"On the 8th I had followed the Army of the Potomac in rear of Lee. I was suffering very severely with a sick headache, and stopped at a farmhouse on the road some distance in rear of the main body of the army. I spent the night in bathing my feet in hot water and mustard, and putting mustard plasters on my wrists and the back part of my neck, hoping to be cured by morning. During the night I received Lee's answer to my letter of the 8th, inviting an interview between the lines on the following morning. (Correspondence from Grant to Lee followed)
I proceeded at an early hour in the morning, still suffering with the headache, to get to the head of the column. I was not more than two or three miles from Appomattox Court House at the time, but to go direct I would have to pass through Lee's army, or a portion of it. I had therefore to move south in order to get upon a road coming up from another direction. (Correspondence from Lee to Grant received)
When the officer reached me I was still suffering with the sick headache, but the instant I saw the contents of the note I was cured." (Correspondence from Grant to Lee followed)
At this point, Grant states he had "no doubt about the good faith of Lee and pretty soon was conducted to where he was".
Grant comments on Lee's dignity, and recognizes his army fought long and valiantly in the circumstances, also acknowledging they suffered much. Giving his perspective on the cause he, at the same time, does not question their sincerity.
Given the import of the moment, it seems the matter of dress was the furthest thing from Grant's mind - "this was not a matter that I thought of until afterwards."
I think we could learn a lot from observing both these men on that day, and I could say so much more. In some ways I feel it is a defining moment in the conduct of the war and war in general. Grant felt no rejoicing and refused to humiliate Lee's men any further. This is an abject lesson in humility from Grant also. I find so much to admire about him as well. But, I'll stop there.