I'll be gentle
This phrase is always a setup. It means they're about to do something super rough. It's like when your dad says 'I'll be quiet,' and then he yells the whole time.
'I'll be gentle,' he said. Then he pulled my hair and kissed me roughly.
'I'll be gentle,' he said. Then he tackled me and pinned me down.
'I'll be gentle,' he said. Then he shouted in my ear and made me blush.