Set default meta tags and overside within controller or view

What would be the best way to set up default meta tags globally.

And then where you want override them in the view or controller action.

I think that layout is only place where you can set meta tags globally.

And you can overwrite them in controller action or view for that action.

Maybe I’m wrong but I think that $this in controller action and $this in corresponding view file(in case there is view file) is same variable.