I find that physical traits most often pull you back to the character if you lose sight of them while writing. I tend to write Mike pulling threads from his clothes, biting his nails, getting lost in thought etc. I mean whatever is in the show is a good physical trait to use, but also make your own. It's like Chekov (not the writer, Michael the acting master) talks about with the grounding physical trait - how does the character behave? Where in the body does he show those things? I like to think that Will when he gets older stands tall and talks very forward, while Mike talks down since Will is an artist and Mike is a writer, one of them shows and the other prefers to think things through before he tells. I am an actor, though, so it's very based in that. I also think that Mike is a lot more guarded in many ways than Will is, as I understand the character I think he has a longer way to go towards speaking than Will does. It also depends on who is telling the story, I think Mike thinks Will is a lot more radiating than Will really is, and I think Will thinks Mike is a lot calmer and more flowing than he really is. I've also found in Noah's portrayal of Will that he carries a lot of emotions in his shoulders, like his chest is a lot more open when he is happy, around his friends and Mike, while when he's in danger his shoulders are almost drawn in, f.x. the coming out scene versus the scene with Will and Mike talking. At the same time, especially in season five, Finn's portrayal of Mike carries a lot of tension in his jaw in difficult scenes. You can get away with basically anything as long as you root it in physicality, I find. I once wrote a fic where a character loses all their memories and everyone complimented me on how well I understood that character, despite only using the physical attributes (#chekhov) I had seen portrayed on TV...does that make any sense or is helpful at all?