+0
I don't know when is right to used one or the other.
Example: I received a letter from her. I was surprised, because I wasn't waiting/expected
it.

Thanks,

Madeira
Comments  
'To wait for' and 'to expect' are interchangeable when they
mean 'to look forward to the probably occurrence of'.
Then, both sentences have the same meaning:
- I was surprised because I wasn't waiting for it.
- I was surprised because I wasn't expecting it.
"I was surprised because I wasn't waiting for it" sounds very odd to the native speaker, so I would try to use "expecting." However, it is correct.