Characters become round through betrayal, and then acceptance of their shadow. This is a difficult feat to pull off, and it is taxing on the reader, so it must pay-off. Due to the energy expended in round characters, most characters must necesarily be flat.
So, make flat characters pleasing by making them comfortable for the reader in predictability and identity. Make the flat characters that quirky person you met in college and still remember. Each flat one should have “flat memorability.”
Round characters consistently surprise you. If they are not surprising – then why bother?