Couples who roleplay, are you able to stick to the characters?

My wife and I (married 14+ years) have an active sex life and over the years have shown interest in role playing. However whenever we have tried, we have not been able to stick to the characters and end up bursting out laughing and then have sex or just have sex as usual with the only difference being the outfit that we start off with.

For the life of us we have not been able to even use one line that we normally find hot in any reference which got us interested in that specific roleplay to begin with. I want to know from ITS how (if at all possible) they have been able to stick to the characters?