In the very first paragraph, Lovecraft gives us this description of Crawford Tillinghast:
It is not pleasant to see a stout man suddenly grown thin, and it is even worse when the baggy skin becomes yellowed or greyed, the eyes sunken, circled, and uncannily glowing, the forehead veined and corrugated, and the hands tremulous and twitching. And if added to this there be a repellent unkemptness; a wild disorder of dress, a bushiness of dark hair white at the roots, and an unchecked growth of pure white beard on a face once clean-shaven, the cumulative effect is quite shocking.
Lovecraft is able to give us such a dense physical description without disrupting the story by making the description itself illustrative of the action within the narrative. We know, per the text, that Crawford Tillinghast has spent over two months shut up in his laboratory, but this physical description gives us a deeper look at what's been happening behind closed doors.
Tillinghast is thin and his eyes are sunken, suggesting that he's been so fixated on his work that he's not eating right, not sleeping enough, and probably not paying much attention at all to the basic necessities of human life. His skin has changed color—he may be sick, he may be malnourished, he may be so starved for sunlight that his skin tone has completely changed. He's unkempt—not only is he neglecting his physical health, his outward appearance in general is of no interest to him. He's not even keeping up basic appearances.
As I entered the abode of the friend so suddenly metamorphosed to a shivering gargoyle, I became infected with the terror which seemed stalking in all the shadows.
This description—also of Tillinghast—is a little more subtle. The narrator is describing the setting. By likening Tillinghast to a gargoyle in passing instead of refocusing the narrative on him directly, he makes the man part of the scenery. This allows Lovecraft to emphasize Tillinghast's dwindling humanity while also maintaining the momentum of the story.