Smiling VS having a Stern Face

I have had various slaves that I session with and film custom clips for. From time to time, I get two different desires from subs:
I'll have subs that say they like when I laugh and smile at them, like a bubbly fun attitude.
Then, I'll have other subs who will say they like when I have a serious/stern or "bitch face" look.
I often do whatever I feel, and express however I feel (which is how it should go). Now, I don't mind smiling/laughing or being serious. Either way, I enjoy it.
For filming customs, I do have a few slaves who prefer me to be very stern in the clips & it does get hard to not laugh at my slaves humiliation requests... & I get some very funny humiliation requests. When it comes to humiliation, I feel like it should always be funny for the Domme and she should always get entertainment out of her slave doing some crazy task.
So, I want to hear back from both Domme's and sub's on this. If this has ever happened to a Domme? What do they prefer?
& for subs, do you have a preference? Does it really matter?