The ending of the "Two Brothers" trailer seems to suggest that the lines are mostly improvised, as the narrator appears to be surprised by his own (so surprised that he laughs and moves off mic to remark on how ridiculous he finds what he just said), but this isn't necessarily an indication that all of the dialogue is improvised.
http://www.youtube.com/watch?v=9ziXpIPAhD4
In fact, <PERSON> (co-creator with <PERSON>, and the voice of many of the characters, including <PERSON>), says he reads <PERSON>'s lines once, then marks down where he wants to insert <PERSON>'s trademark burps, and then goes back and re-records the dialogue.
The burps are real. While doing all of <PERSON>'s dialogue for an episode I mark specific lines that I want to go back and do "burp takes" on. I have to tell the Audio Engineer (<PERSON>) to leave it recording. I use low calorie beer (miller 64) and water to get air into my stomach. Then wait for the burp and try to get it in the middle of a line. It's disgusting reading this explanation I just typed. This whole process is disgusting.
- Source
http://www.youtube.com/watch?v=xxkuayMXBOI
However, most of <PERSON>'s lines are indeed improvised, as he revealed in a question and answer session:
When doing the dialogue I have to toss the script aside. Even if I'm sticking to a very specific line of dialogue that is required for a very specific joke set up or punchline, I find that if I allow myself to adjust the wording and let it come out more naturally to how I would actually say it, it helps make the characters feel more real/alive/natural. I also LOVE using out takes as in takes. Our dialogue editor <PERSON> also always surprises me with stuff he'll pick from the record to cut in. He and I are very much in sync on what we find funny.
That catch phrase thing is a joke on stupid catch phrases. Seems to be a big controversy at this point. 233
Go back and listen to when he first says it at the end of the <PERSON> episode. He changes it each time he says it. Then in [the] sex robot [episode] he says he cemented it back then. It's the joke. That's the joke. It's a horrible cringe worthy catch phrase that I just made up in the record as an out take that made it's way into the show. CAN'T WAIT TO SEE IT TATOO'D ON A HOOKERS ASS!!!
[I improvise on] some episodes more than others. I usually have moments in each episode that weren't in the script. One big example is the cold open in the pilot. That whole sequence in <PERSON>'s ship was improv based on a scene suggestion from <PERSON>. Other stuff when <PERSON> acts strange or fucks around... that's usually improv. The scene where he plays with <PERSON> in the Alien Scammer's ship was improv.
It is very clear that the episode you mentioned is one of the most improvised episodes so far, as can be seen in the following clips:
http://www.youtube.com/watch?v=iVgMFY1JFGU
http://www.youtube.com/watch?v=-o0Qp5J_Oo8
Update:
More obvious improvisation:
https://www.youtube.com/watch?v=eMJk4y9NGvE
Note: Some guidelines for recognizing improvised material on the show:
* Obvious pauses where the voice actor (usually <PERSON>) is clearly struggling, trying to figure out what to say next
* The same word being used multiple times in a scene, but changing slightly each time (see the "Strawberry Smiggles" commercial, where the produce is referred to by at least three different names). 167