Formal:
* Respectful: This emphasizes politeness and deference.
* Deferential: Similar to respectful, highlighting a formal sense of subordination.
* Polite: A general term for courteous and considerate behavior.
* Submissive: Implying an acceptance of authority and a lack of defiance.
Informal:
* Serious: This implies a lack of humor or playfulness.
* Straightforward: This emphasizes honesty and directness.
* Innocent: This suggests a lack of guile or mischievous intent.
* Boring: If "cheeky" implies a playful and entertaining quality, the opposite might be boring.
Other possibilities:
* Dull: This is similar to boring, but with a stronger emphasis on lack of excitement.
* Serious-minded: This highlights a focused and responsible approach.
Ultimately, the best opposite for "cheeky" depends on the specific situation and the nuance you're trying to convey.