On the turn it is selected, Skull Bash will raise the user's Defense by one level. On the following turn, the user will attack. Once Skull Bash is selected, the user won't be able to switch out until it is disrupted or fully executed.
If Skull Bash is not fully executed, PP will not be deducted from it, and it will not count as the last move used. If the opponent uses Mirror Move during the turn that the user first selects Skull Bash, Mirror Move will copy the move that the user executed before it (or fail if it can't).
Sleep, freeze, partial trapping, and flinching will pause but not disrupt the duration of Skull Bash.
In Pokémon Stadium, Mirror Move will copy Skull Bash on either of the turns it takes to execute.