When you are planning your acting shot it can be too easy to only focus on the obvious acting choices and forget the secondary actions which really accent the characters thought process:
A funny, if not awkward, clip. Now I won't claim David Schwimmer and Jennifer Aniston are the greatest actors is the world but there are wonderful choices here that they do with they body language to further their characters' thought process. Watch the video again, but this time focus on these points you may have missed the first time (skip to the part where he first says "you're over me?")
1:30 - "When... were you... under me?" - Ross shifts his weight left then right (actually stepping both times) during the pause to brace himself for the question he never thought he would have to ask. He also never blinks until the last two words thus ending his chain of thought (then saying it). Also, the pause is a great example in timing. This line could have easily been thrown away with a quick delivery and a long stare.
1:55 - "feelings for you" - even the 20 seconds before the Rachel is fidgeting and can't stay still. But specifically her fingers during this phrase are highly active, she is trying to distract herself and is in a highly uncomfortable situation.
2:06 - "...for me first" - Rachel never looks at Ross as her uncomfortableness leads to anger. She even looks away as she uses her hand as a proxy to "talk" to Ross. When he returns in kind she finally has enough courage to look at him which then breaks his concentration forcing him to look down. His head overlap during the point is a great accent. Again, notice the left-right body shift in both of them before the speak an uncomfortable line. Ross even adds a shoulder flinch to accent his line.
2:20 "..whendidhe, whendidhe?" Two quick eye darts at the end of this phrase show confusion and unwillingness to accept the situation he is in. Even better is the mouthing of words that are not audible. This is a technique rarely used in animation that adds a lot. Most animators only move the mouth during dialogue, but silent mouthing shows the character is so shocked they can't even bring themselves to force air through their vocal cords (or just don't know what to say).
2:28 - "Julie" - notice the blinks before every movement. A blink is a great way to signal a change in thought without actually saying anything. His mind is racing a mile a minute here especially with the added pressure of someone waiting on him outside.
2:38 - "need to lie down" - personally, I am not fond of the acting choices here. At this point the writers were still trying to show confusion (and add humor) but they've milked it too far and David has already used most of his tricks. He once again resorts to finger pointing which is now overdone (he'll do it 2 more time after this too), and his word repetition is now annoying. Notice this gets the least amount of laughs, even the audience has caught on and is ready for the conclusion. The only positive thing is his movements add contrast to the intense stationary conversation coming up...
2:55 - "are you over me?" - Rachel drops her shoulders twice to show vulnerability. The "answer" is another example of Ross' blinks and eye movement showing his thought process.
3:10 - the finger biting is a nice touch here. It many cases finger biting is cliche and should be avoided (think of every animation with a nervous or scared character biting their nails) but the bite here is so quick it is a secondary action and may not be noticed at first but is definitely felt.
As an animator (or actor) these are the tricks that are not included in the dialogue track or written in the script. It is up to you to find these movements to accent your acting. Just don't let them be too obvious (or over used) or the secondary actions will become primary actions which are just distracting. A good secondary action is one that is not seen but felt (although on further viewings they become more visible). How many of these actions did you notice the first time?
And as a bonus here is a video that adds a little more closure to the scene above (with its own share of awkward poses):