Most clowns I know don’t speak at all. But certainly the ones that do, they’ll talk whenever they like if you give them the opportunity. That is part of the pleasure of them; They are anarchic and unpredictable. I wrote a play in which I wanted that sort of energy. I tried to write it into the character but it doesn’t really translate into words. In order to give it that clown feeling, I had to specify that the actor was welcome to stray from the text when he felt like it.
It’s tricky though, giving clowns free rein. In another play, I gave an actor space to improvise in a couple of key moments. I found, though, that he made something up one night and then just said that same thing over and over again afterwards. I ended up writing him a thing because, while what he’d made up was mildly amusing the first night, it ceased to be amusing with repetition.
But then, too, clowns often thrive in adversarial relationships. Tell a clown he shouldn’t say more than is set down for him and I can almost guarantee you that he will be improvising his balls off, all over your script. Tell him not to swear, there will be a mountain of swears by the end of the night.