serious

and

nothing

special

The meaning is very different and should not be translated into the same Chinese.

Your example obviously can't interchange these two phrases. The sentence says, "I talked to my friends quietly and just chatted." If it is nothing.

Serious, it becomes a "not very serious conversation." Small talk can be serious.

We can't understand the original meaning of English with the meaning of Chinese translation, but we must understand the original meaning of English.