How to portay inner dialogue/thoughts?

(Hope I'm using the correct flair) I've tried to research this prior but I feel I don't really get what I'm being told.

The only way I actively know how to portray what a character is thinking always feels comedic or through facial expressions.

I don't mind doing the facial expression route, but I'd be concerned that certain viewers or people reading the script wouldn't exactly be able to realize the unspoken thoughts being alluded to.

What could I do to put character's thoughts on paper/the screen without it coming off forced or comedic? How would I write it out on the script? Any advice is helpful!