I’m so flattered that you want my headcanon *cries* So I’ll do Nicolas now, Armand in the next one ;] Previously: Louis.
Same disclaimer: #your headcanon may vary.
Nicolas is often compared to Louis, and I do see them as both being thin men, almost 6 ft tall, not huge in the muscle department, and they have similar hair color and texture (dark and wavy to their shoulders). They also both carry themselves in a fairly dignified manner, they both have long legs and yes, nice butts. But the differences are very apparent when:
A) You are within a few feet from them: Nicolas is more fidgety, often crossing and uncrossing his legs, changing positions, gesturing more when speaking. He has wider shoulders than Louis, and is small-waisted due to a high metabolism in life, he ate an enormous amount of food w/o any consequences. Not athletically inclined, just great genetics (altho I picture his dad as very barrel-shaped, which was part of why he liked to dress his son up so much, better model!). Conversely, Louis was malnourished and the quality and amount of what he was willing to eat/drink would have ended him if he hadn’t found someone else to do it first.
B) You take off their clothes: Nicolas’ high metabolism and hearty appetite (even in Paris, he was eating that great French bread and wine) made for muscles like a racehorse. More for savory than sweet; I don’t imagine Nicolas as having much of a preference for pastries, which he would eat only bc Lestat was something of a foodie and brought treats home now and then.
I would also imagine Nicolas is darker-skinned than Louis and Lestat, and this complexion further defines his muscles, especially when his skin is wet, all those sharp edges really shine ❤

[X] ^I couldn’t find a good human body as an example so here’s a thoroughbred. Not this dark color for his skin, example is for the muscle definition. Nicolas would not have been a horse that would let many ppl ride him, tho.
C) When you get very close to them: Nicolas has a sprinkling of birthmarks that are not mentioned in canon, which form constellations mainly on his face, neck, chest, and arms. Louis might have these too, but a lot less. Lestat liked to trace connections with his finger when they were in bed together and Nicolas secretly loved that ❤

^Kylo Ren has these facial birthmarks.
Also, differently from Louis’ blazing green eyes: I don’t think Nicolas’ eye color is mentioned in canon, EDIT: “His brown eyes were very big and trusting.”… “large sober brown eyes” I always pictured them as that great light brown w/ dark limbal rings that absorbs all the light in the area, almost gold in sunlight:

[X] He also has long eyelashes, longer than Louis’ eyelashes, and along those lines, for performances at the theatre, I headcanon that he let Lestat apply some guyliner for dramatic effect ;] Not that he needed it, but that he liked Lestat giving him that kind of special attention.