When it comes to improvising and short comic strips:
Personally, I don't really plan ahead of time when it comes to strips since the main thing is the punch line. So, something simple and sweet. BuUUTTTT, if it's a comic with story then boy, get all yoooo' papers, get yoooo' peeennss, get yooo' wifeeee, plan it all and write tons of scripts you will most likely cross out and overwrite cus' it's hell. Hahaha
Then see how the panels fit with the script. It's a lot to think about. Improvising is fun (And I admit it's what I do most of the time) but you also need to keep an eye out not to regret it later. (Which I don't wanna admit but have done countless times)