Xiao Zhan (Wei Wuxian)'s character originally wore a high ponytail, but this detail was forgone seeing as how the actor was already taller than Wang Yibo (Lan Wangji). Lan Wangji was supposed to be taller than Wei Wuxian. The directors even styled Wang Yibo's hair to make him look taller, but could not do so anymore in fear of looking ridiculous.
Most actors have their voices dubbed over; only a few use their actual voices. In China, there are a lot of regions and so, there are so many dialects. But the Chinese words are the same. (This is also the main reason why there is Chinese subtitle even though they are talking in Chinese.) So, in order to be standardized, they use voice artists for dialogues. This is very normal for Chinese TV series.
In the first place, Xiao Zhan didn't think that he and his character Wei Wuxian weren't quite similar that much. Maybe because later, he merged himself into the character Wei Wuxian for filming, so, during filming time period, he became like Wei Wuxian. He became very talkative and teased the other casts and crews.
Wang Yibo's character, Lan Wangji didn't have to talk that much unlike Xiao Zhan (Wei Wuxian's cast). He only had to express everything with looks. So, Wang Yibo felt awkward during filming.
Mo Xiang Tong Xiu is a Chinese author well-known for her danmei (the Chinese equivalent of the "Yaoi Genre"=Yaoi, also known in Japan as "Boys' Love", is a subgenre of Shoujo focusing on male homosexuality, for a predominantly female audience) novels, and is one of the most successful authors of the genre, with her three (currently) published works receiving an Animated Adaptation.