Couldn't we use mixins in run-time?

[rempas]

I'm really wondering, what stops us about using mixins in 
run-time? I'm really interested on learning how they work. For my 
understating, the write the code for use in compile-time and then 
the whole things is compiled. However, I'm really wondering what 
stops us for writing the code and compiling it at run-time like 
it's an interpreter. Anyone that can explain how things work to 
me?