A display of emotion is compelling both in real life and in fiction, and it is well worth designing a narrative so that characters have emotional highs and lows. This is not to say they should be going from one extreme to another, but that on occasion they are seen to be humoured or annoyed. Doing so will make a character feel more relatable and authentic.
Without displaying a range of emotions, a character risks being dull; a quality that is especially undesirable for a central character, who plays the most important role in maintaining reader engagement.
Showing a character’s emotion needn’t require any restructuring of plot, as they can often be introduced into an already established scene. Look for instances where a small shift in emotions may occur for a particular character. Humour often arises after a period of sustained effort, such as after a long journey or some hard labour. Annoyance is often triggered by stress, which can arise when time is limited or when something unknown must be undertaken (for some characters, this could be something as trivial as driving on an unfamiliar road.)
Here is a moment of brief annoyance, seen through a character’s complaint:
They got the big tent up on the Forest Service’s platform, the kitchen and grub boxes secured. Both slept in camp that first night, Jack already bitching about Joe Aguirre’s sleep-with-the-sheep-and-no-fire order, though he saddled the bay mare in the dark morning without saying much. (Brokeback Mountain, Annie Proulx)
And here is a moment where a character briefly displays joy:
She laughed and spun around and gave me a look of her yellow leg when the skirt billowed out. She threw me a wink over her shoulder as she ran back to her machine where a pile of fiber was spilling off the table to the floor; she grabbed it up and ran featherfooted down the aisle of machines to dump the fiber in a hopper; then she was out of sight around the corner. (One Flew Over the Cuckoo’s Nest, Ken Kesey)
Small displays of emotion such as these go a long way in humanising characters, lending authenticity to their persons and adding emotional texture to their tale.