Warning: this is not finished and may be updated at random points

Pupils/eyes

Dilation
If one is somewhat attracted to another, staring into said persons eyes can cause the pupils to dilate up to 20% of their original size (also the palpitations of the heart increase but see more in the later topic 'heart'.

Eye contact
If a person is conversing with another, and said person is not maintaining eye contact, this could indicate that they are somewhat uninterested in the conversation or bored- however, it could also mean that they are nervous, or anxious, which could be linked to whom they are talking to; or what the subject of the conversation is. If the pupils are dilated (mentioned above) then this means that they are possibly anxious, but fond, most likely about who they are talking to, or who/ what they are taking about (usually a person). Glancing at other items can show what they desire, such as a door, or another person indicating they want to leave, or talk to someone else. Looking to the upper right suggests that they are lying, as the lower right part of the brain is for imagining and creating stories/situations- whereas looking up to the right indicates the truth is being told since the bottom left side of the brain is recalling a memory.

Blinking
A persons rate of blinking can increase heavily while: lying, stressed, etc... An especially prominent feature of lying can be the excessive touching of face, and adjusting of posture.

Facial

Expressions
A genuine smile takes up all the face and muscles, and usually creases the eyes, while a fake smile usually only involves the mouth area, and is often followed by a short laugh that is not usually accustom to said person, and sounds different from their natural laugh. Half smiles engage half of the face, and often indicate uncertainty, or sarcasm- or even a joke or something spiteful that has been said about you that you don't know of yet. Some may scowl before smiling which can mean that they do not want to be in your presence, the subject of the conversation is annoying them, or they do not want to talk to you. Tight, pursed lips indicate similar.

Proximity/ body

Movement
The distance of one person to another is very important as to understand body language, as it can reveal how they feel about person a. If someone moves closer to you upon entrance, this means they view you positively, and feel fondly about you- however, if a person shifts uncomfortably or backs away, this could suggest they have been talking about you, or do not like your presence.

Mirroring
If a person were to mimic a small action that you commit, such as crossing your legs, within ten seconds of doing said action, mean that the could: look up to you, be trying to get closer to you, seek your approval, or trying to form a rapport relationship with you.

Head movement
The movement of a head is an important aspect of reading body language- for example the slow nodding of a head while in a conversation, mostly consisting of listening to the other, can indicate a genuine interest in the others speech, and willingness to listen- however if it seems exaggerated, it means that they have lost interest, and are not listening or invested. Fast nodding, can also mean similar- as it means that they would like you to finish talking quickly, and stop. Tilting your head backwards can reveal uncertainty, or disbelief in what the other is saying.

Feet/legs
If a person is pointing their feet in another direction, while not letting on any other clues about their feelings, this can show where they want to go. For example, if said persons feet are pointing away from another, this can mean that they want to leave that way, and are ready to do so quickly after you have stopped talking. If I'm a conversation, and the feet are pointing in another direction at a person, (feet if standing, legs if sitting- this does not include crossed legs) they would rather talk to them.

Hand movement
Hand signals are often subconsciously made, and can suggest a lot about a person and what they are trying to say. Many sharp hand movements can suggest frustration and trying to get a point across. Using an elbow to prop up the head suggests they are trying to focus more on the person, or they are bored, which can usually be noticed by the addition of other body language. Holding an intel in front of themselves serves as a barrier meaning they do not want to engage in further conversation. Folding arms around stomach can indicate the feeling of being unsafe or anxious, since it is natural to protect the primary organs. Crossed arms and a smile indicated confidence and happiness, unless the smile can be perceived as fake.

Physical

Touch
If a person is very touchy and physical while in a conversation, it can imply that they are very close to you, and like the sensation of touching you- or they are every invested in the conversation. This could be platonic, or something else, which can usually be deciphered by said persons other body language that they display.

Heart
This is a more difficult one to read, since it is hard to inconspicuously read someone's heart rate, but if the chance arises, this is what it means. A quickened pulse while talking to you, means that the person is interested in you, possibly romantically, or is enjoying and is invested in what you have to say. If the pulse stays at a normal rate, it can indicate the person is more casual, or is bored by the conversation that you are making- but this isn't usually the case if you are having a simple conversation, a normal pulse isn't irregular.