"Attentiveness gets not infatuated with the good feelings nor sidesteps the bad as attentiveness is a non-feeling awareness; a sensuous attention. Attentiveness is not sentimental susceptibility for it does not get involved with affection or empathy or get hung up on mercurial imaginations and capricious intuitions or ephemeral auguries. Attentiveness does not register feelings and compare the validity of experience according to it ‘feeling right’ or ‘feeling wrong’. Attentiveness is an aesthetic alertness that takes place with minimised reference to self. With attentiveness one sees the internal world with blameless references to concepts like ‘my’ or ‘mine’. Suppose there is a feeling of sadness. Ordinary consciousness would say, ‘I am sad’. Using attentiveness, one heedfully notices the feeling as a natural feeling – ‘There is human sadness’ – thus one does not tack on that possessive personal concept of ‘I’ or ‘me’ ... for one is already possessed. Attentiveness is the observance of the basic nature of each arising feeling**; it is observing all the inner world – emotional, passionate and calentural – which is whatever is presently taking place in the affective faculty. Attentiveness is seeing how any feeling makes ‘me’ tick – and how ‘I’ react to it – with the perspicacity of seeing how it affects others as well. In attentiveness, there is an unbiased observing of the constant showing-up of the ‘reality’ within and is examining the feelings arising one after the other ... and such attentiveness is the ending of its grip. Please note that last point: in attentiveness, there is an observance of the ‘reality’ within, and such attention is the end of its embrace ... finish.
Here lies apperception." Richard

Question:
What is the difference between a constant mindfulness of the body and it's experiences, emotional, physical, and intellectual, and attentiveness??
Is it just that attentiveness includes an investigation into affective feelings?

** As far as I understand this highlighted sentence, it means to be attentive to the very 'basic nature' of one's experience in any given moment. And at any given moment a sense door is being hit with a sense object and out of ignorance and lack of apperceptive awareness, mental proliferation follows and gives rise to affective feelings and a sense of 'being'/me-ness/inner world/presence/location in the world. If you are attentive to the point of contact of a sense object and its corresponding sense door, the sequence that usually out of ignorance follows the point of contact onto the arising of an affective filter/sense of 'being' will be interrupted. Doing this continuously will then eventually lead to apperception (PCE: the temporary abeyance of mentioned sense of 'being' etc.). If it helps to equate 'mindfulness' to 'attentiveness' then do so.

The original OP was asked from the Insight and Wisdom forum, which presumably is meant to be within a Buddhist context (hence my question to the OP about "perspective"), and from not the AF or Actual Freedom subforum. Had it been asked from the latter, I would not have responded, as in my view AF is an unnecessary, superfluous, and distracting practice. Although for those who feel they have benefited from it, I say good for you, I wish you the best with that.

"Then, Bahiya, you should train yourself thus: In reference to the seen, there will be only the seen. In reference to the heard, only the heard. In reference to the sensed, only the sensed. In reference to the cognized, only the cognized. That is how you should train yourself. When for you there will be only the seen in reference to the seen, only the heard in reference to the heard, only the sensed in reference to the sensed, only the cognized in reference to the cognized, then, Bahiya, there is no you in terms of that. When there is no you in terms of that, there is no you there. When there is no you there, you are neither here nor yonder nor between the two. This, just this, is the end of stress."

http://www.accesstoinsight.org/tipitaka/kn/ud/ud.1.10.than.html