I sort of explained that formal speech is generally used when there is some distance between the speakers, respect for the other, and or stance to be maintained of the speaker. Informal speech is generally used when there is a closeness between the speakers, a social order in play, or a lack of respect for the other.
I might say 'Hey, John. What's up?' at a meeting if John was my close friend, regardless of his status as my boss. If John's superiors are around, I might distance myself to show respect for him. So, situations have many variables.
- At the dinner table, I ask my son, 'Would you please pass me the ketchup?' Not, 'Hey! Gimme the ketchup.'
- However, when he climbs up on the table to get a cookie, I'll shout, 'Get down. Now!'
The first example being that I try to be polite to show him respect and to teach him to be respectful. In the second situation, the level goes down to point out he is out of place and doing something wrong. (I could ask him politely, but it's the 15th time, for crying out loud.)