I mean, he would be such a fun, complex character to play, right? He's funny, brash, arrogant, unapologetic, and dangerous... It's all very attractive and exciting. And so far, they've made him appear very human and sympathetic. I can totally see why Megan would want to play him.
I bet it will be even more interesting after Darnley and Mary are married and he flips the switch.
When they were racing their horses at the beginning of the last episode and he told Mary that he "let" her win the race, but that once he is King of Scotland "I will have my way." YIKES. It's fun banter, but it's also very ominous.