On your turn, you misty step 30 feet straight up. Can you make an attack before you start falling, or will you immediately begin to fall?
For example, a player wants to reach a flying enemy to knock him prone, then feather fall safely to the ground. Is he in reach long enough for all his attacks to go through? Will he fall before he can cast feather fall? (He used his bonus action on to cast a spell, so only cantrips during his turn.)