[*Flat*](http://www.thefreedictionary.com/flat) usually refers to dull and emotionless.

>lacking interest or emotion; dull and lifeless; lacking vitality or animation. 

It is often misused, I had to look for a while to find a good example, and this was the best I could do (it's not even a good sentence!)

>Maybe because his *flat eyes* had no expression and he was squinting at me like a mob character. 

Flat can similarly refer to *affect*, *scenario*, *food* or *joke*.