We tend to not crossover registers when speaking, as this could make our intended utterance come across as incoherent.
Formal words or terminology in informal expressions are usually only used when trying to emphasise a certain tone.
In the example below, a mother is talking to her son, emphasising that she is annoyed at his lack of activity/lazy attitude:
Do you think you might be able to remove your bottom from the sofa and clean up your bedroom please?
Formal words can usually be used in informal contexts to denote sarcasm, irony, irritation, humouring and emphasis.
In written language, we might opt for formal wording more-so than with spoken language, even if writing to someone we would generally speak on informal terms with.
