I have moar questions for the OTP + CHARACTER ASK because I really wanna pick your brain (only if you want!!). So: 16, 18 and 19 C:
bestie, i love when people pick my brain.
This one is so straightforward to me. Zeb is covered in fur and rarely needs covers (until Hoth). Kallus has minimal fur and frequently feels like the chill never left him after Bahryn (this I think is why he's still wearing a quilted jacket on the humid rainforest that is Yavin IV). It's less that Kallus hogs the covers and more that Zeb gives them to him and then wraps around him too (until Hoth, when Zeb valiantly tries to steal some back, or at least cuddle close enough to Kal that they can reasonably share the covers).
They're both big lads so I think they both have a pretty high tolerance for how much alcohol they can consume. Where they differ is Zeb still has a few stone on Kallus, so he can drink more without feeling it as much, but Kallus is also incredible at still appearing sober when he's deep into his cups. Kal started drinking early and his time rising through ISB meant he got really good at lying, so if you aren't super close with him you may not even realize the glass in front of him is number 4 and not him still nursing the first one. But those who are close to him, namely Zeb but the rest of the Ghost crew too, can pick up on little tells. He laughs a bit more freely, he's even snarkier than usual, sometimes he gets flushed. Because he's an adult, he knows not to get into the rowdy shenanigans he used to as a teen, but sometimes there's a glint in his eye and he gets tempted to pickpocket or climb the side of the temple or drag Zeb to dance with him (see: makeout on the dance floor). If he's playing sabacc, though, he lets his opponents think he's way more drunk than he is and swindles them out of their credits.
Zeb gets more outgoing and gregarious, which is a freedom that comes with joining the Rebellion and having a relatively safe home base. Every time he's drunk he makes new friends, and for a lot of those new friends it's the first time they see Zeb as someone who's approachable and not scary. He's relaxed and has his arms draped over the back of the booth, but when the mood strikes him he can get rowdy and wrastle with people. He never starts a fight though.
The two of them drinking together is honestly not much different than them relaxing at other times. They're old men (by Rebellion nightlife standards); they aren't trying to give themselves hangovers tomorrow. But they will sit shoulder-to-shoulder for an entire night, having quiet side conversations and making the rest of their table think they're already together when neither of them has as much as thought about it.
When only one has been drinking, both have been known to fireman carry the other back to their bunk.
Before they're together, it's really small stuff mostly (I know you used my repair kit, now where did you karking put it), but sometimes it's really big stuff, along the lines of who is allowed to forgive whom and for what. Kallus doesn't feel worthy of Zeb's forgiveness for Lasan; Zeb doesn't feel worthy of Kallus' forgiveness for not getting him off the Chimaera sooner. But they work through it. They talk to each other, even if they go in circles for a while, and they trust each other.
They're later fights are even more hypocritical. They always yell at each other when there was a close call or when one of them did something reckless, even though they both know they would've done the same thing if it meant saving the rest of the ship or not sacrificing the mission. Oooohhh but these can be knock down drag out fights. They say horrible things to each other because they know each other so well and know their pressure points, then immediately regret it. These fights don't happen often, but they require a cool-off period. Zeb stays with Hera, Kallus stays with Cassian or Sabine if she has her own quarters. But even after a fight like this, they still can talk it out and repair it.
On Lira San, the fights go back to "where did you put the damn vegetable peeler."