I think Jayne is an ISTP, like Clint Eastwood or Steven McQueen. This type relates to the external world through extroverted sensory confidence (in these cases, as a gun-slinger) But their strongest personal characteristic is internalised logic. Note, that doesn't mean the person has to be very brainy, it just means they are unsentimental, focus on what is rather than what ought to be, and this is their strongest attribute. Their weakest attribute is feeling - they don't know how to express it or deal with it.
I think executrix sees Jayne through quite different coloured specs than I do (ie she can't stand him). I guess that she would call him an ESFP like Vila Restal in Blakes 7. An ESFP, like an ISTP, relates to the world primarily through extroverted sensuality (both Jayne and Vila have a craft skill, and both are quite randy). But an ESFP is more open (his extroverted sensuality is his strongest feature), and has a kind-hearted streak (his second strongest feature is introverted feeling).
This makes me think about what introversion is. Both extroversion and introversion are quite complex attributes, which can manifest in lots of different ways. I think introverts with dominant thinking (that would be ISTP and INTP) can be the toughest minded people of all because their priorities are influenced by neither emotion (F) nor sociability (E).
Incidentally meinterrupted talks us through a scene with Adam Baldwin in a bad film called 'Next of Kin'. Adam is forced to his knees at gunpoint (he's not happy about it) and meinterrupted has plenty links to screencaps. Incidentally the guy forcing him to his knees is Liam Neeson, but you can't see that in the pics, alas.