hi - arriving in UK with fiance visa?

Hi,
I was just wondering, if an american citizen was to obtain a fiancé visa to come and marry a uk citizen, what will the immigration at the airport be like? Will they interrogate my fiancé and reject her if they don't believe her or is it the case that if they have a visa then my fiancé will just be waved through immigration and it just be technicality to check that she is who she says she is?

Thanks

[Moderator edit (John) : History shows that having a subject such as "Hi" leads to fewer responses, so I have added something a little more descriptive.]

Having already got the fiancée visa should be considered the hard part of all this. Arriving in the UK using that visa for the first time really should not be considered a problem at all.

The point is this ... a fiancée visa is a type of settlement visa .... she is expected to ... err ... settle in the UK. About the worst she might be asked is ... what are your plans for marriage? But even then the answer need not be specific ... something like ... we shall go to the Register Office in about two weeks time to give the Notice of Intention to Marry .... and while there arrange a specific wedding date with them ... or whatever ... will be totally sufficient for the purpose.

Has she already got the fiancée visa?