If you decide to live out your fantasy, make sure you're familiar with any safety guidelines, and consider the potential emotional as well as physical effects. With some fantasies, you may decide to use them to inspire talking dirty but leave it at that.
Also, consider your partner's reaction. Just because they're willing to try something, it doesn't mean they'll necessarily enjoy it: don't let your arousal make you selfish and over-ride your lover's desires in favour of your own. Conversely, don't feel obliged to say yes to something you don't really want to do. It's better to be honest – without being disrespectful or judgemental – than ignore your own needs.
Living out fantasies can be fun but if things don't go to plan (or even if they do, sometimes) you may find the power of the fantasy fades in the light of reality. Think about whether you're willing to risk losing a surefire arousal builder for the chance to make your dreams come true.
If you decide you still want to make your fantasy come true, take it slowly and check your partner's reaction throughout. Stop if you see they're not into what you're doing – and don't leave your sense of humour at the bedroom door. Relax, communicate clearly and be honest and who knows, you could add a whole new dimension to your sex life.