My only advice is that the decision to do so should be totally up to you alone. If you think it would be fun to act out a fantasy or even just talk about it, go for it! You can shoot an e mail or leave a note or an erotic story on your partner’s bed if you can’t bring yourself to bring it up. Or, snuggled up in bed when it’s dark and they can’t see your face is always another option – but be prepared, you might have to follow it up with real sex.  But never let a partner push you into telling him or her a fantasy that you want to keep private – they are your fantasies alone and you never” owe” it to someone to tell them your fantasies.