There are usually two scenarios:
1. The hubby doesn't know how to broach the subject and even introduce his wife to his cuckold fantasy and would like to set the wife up by having a pre select bull attempt to pick up on her unsuspecting to see if he can seduce her into cheating or at least being excited by the attempt!
2. The hubby catches the wife at a vulnerable sexy moment like during or after drunken sex and proves her with questions about how she would feel about the swing lifestyle or having sex with a black man while he watches? Probe her for answers while she is in the horniest mood or very buzzed to see what she says? If she doesn't not come off totally offended then you can play around with the fantasy as though you are actually just teasing her about it, but I suggest you do it while she is very buzzed or *******. Hopefully she does ******* or use 420!
Hope that gives you some basic idea of how to open the door!