Which is exactly what Han started as...but he grew as a character...so did Jayne. Jayne even goes as far as to strap everyone else in and make sure they are alright...frantically mind you...as Serenity is falling from the sky before taking care of himself. There are subtle momments in the series and the film that let you know he is growing and caring about the people around him.
That wasn't what I meant anyway...I meant the plain way Jayne speaks, the simple manner...he is very off the cuff, plain speaking...as is Han. Mal on the other hand is witty, and complex...there is a lot going on under the surface...a complexity that Han doesn't have...but the charm and the heart of gold is there...so the balance between those two performances woyuld be about right. Fillion could do it, he just wouldn't be my first choice.