everyone is giving great answers. If you are using a safe word it should always be respected, however if you have practicing BDSM fantasies with a gag,are into choking, or you partner is mute handsingles are also a treasure, as well as being able to ready body language too.
JR's response is very good too,because people are always allowed to make up their mind and "take it back" once consent has been given. so if there is not any sort of negotiation beforehand about a safe word or BDSM practices, you always should respect that persons feelings and be understanding about their wishes.